By Bob Goemans
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Two Little Fishies 

The 'LIVING' Marine Aquarium Manual

Basic and Advanced Husbandry for the 'Modern' Marine Aquarium

by Bob Goemans

Chapter 9 - Water: The Basics

Seems like it has always been said that marine animals are more difficult to keep than those coming from freshwater environments. Yet when one thinks of the many different locales where freshwater animals originate, many having water quality parameters that vastly vary from one area to another, then compare it to those creatures maintained in the marine hobby which mostly come from a very stable reef environment, one that has been in fact relatively stable for millions of years, one has to wonder about the accuracy of such an assertion. Nevertheless, with many decades of experience in both hobbies it has taught me there are two areas in the keeping of marine organisms that do require more understanding than those associated with the freshwater hobby and they are the more complex aspects of 'seawater' chemistry, and that of nutrition. Furthermore, 'nutrition' does not solely apply to eaten foods, as it can also be attributed to the dissolved nutrients, elements and compounds that marine fish absorb through the process of 'drinking' the surrounding seawater or where invertebrates are concerned, absorption via their surface areas.

Unfortunately the lack of adequate knowledge in these areas has, among others, contributed to losses in marine animals over the past few decades that have lent some credence to marine animals being more difficult to maintain. To shelve such thoughts once and for all, let's first dig deeper into the subject of water and the chemistry of seawater, yet do so in a logical and not overly technical fashion that might be difficult to comprehend. And when it comes to overall nutrition, probably right behind this topic in importance, it will be discussed in detail in Chapter 14.

As for every aquarium, no matter what its size or complexity, it consist of various building blocks so to speak, e.g., the physical aquarium structure; biological processes; lighting; filtration equipment; quality water parameters; and different kinds of substrate to name some. And depending upon the expertise and budget of the aquarist, many of these building blocks will vary in their quality or application; nevertheless, if the organisms in this aquarium are subjected to poor water quality their health will decline. In addition, the aquarium environment will also no doubt diminish, and very possibly the expenses associated with all the building blocks may go to waste, including its organisms.

The more we learn about water and its various factors, the more we can control its quality, which is paramount to the health of the animals it surrounds.

And before the important aspects of seawater and its use in marine aquariums are discussed, think it's necessary to begin here with an examination of water in general. Then, equipped with those important basics it will hopefully help to understand some of the complexities associated with seawater parameters that is discussed in the following chapter. Then finally in this Section, discuss its maintenance in the following chapter.

Should also note before going further, depending on who's column/articles/book is read, you'll see 'ppm' (parts per million) or 'mg/l' (milligrams per liter) used to note concentration levels. For all practical purposes they are the same and have decided to stay with ppm throughout this book.

(Please keep in mind all underlined word(s) are linkable files - just click on them and be taken to its content/photo. Also, all shown photos are clickable, which often allows a larger file to be seen.)


Water has received more scientific and technological research than any other substance on Earth. That's quite understandable since water is the most common molecule on its surface. It even makes up about 70 - 80% of the human body and the food we consume. It's therefore understandably essential to life and fortunately very plentiful, as there are almost no areas on this planet where it does not exist in one form or another. Nevertheless, it commonly remains one of the most blindly accepted and poorly understood phenomena on this planet.

Basically, the water molecule is composed of one negatively charged oxygen atom and two positively charged hydrogen atoms, and is often referred to as H2O. These water molecules are properly called polar molecules (Dipole) because the opposing charges are located at opposite ends, such as with the common magnet. When seen as water, the positive end of one molecule is weakly joined to the negative end of another molecule and said binding attraction is called a 'hydrogen bond.' As stated, this is a weak bond and is constantly breaking and reforming and what is seen is a unique product called 'water.'

Depending upon temperature, when it loses heat energy it contracts, however unlike other liquids it expands when completely frozen (ice). This is why ice floats as it now has less water volume than an equal amount of liquid water, and therefore is lighter. And this is the reason lakes, rivers, and seas do not freeze from the bottom up, which would kill any organisms growing on their submerged surfaces. Another important aspect of water is its capacity to dissolve other substances. In fact, water is often referred to as the 'Universal Solvent.' Water also has other fascinating properties, e.g., an electrical charge can still be found in the purest form of water ever created by man, and still has scientists baffled as to why this occurs. And there's its high surface tension along with other unusual properties that continues to fascinate scientists. Where aquarists are concerned, they can find it discussed to some extent in every book printed about aquariums. Many aquarists simply consider it the carrier or medium that is used to sustain the organisms in their aquaria. But there is much more to it!

Drinking Water

Have you ever given much thought to the quality of water that comes out of your faucet? Probably not, as most people take it for granted. But as a retired environmental contracting manager I've learned you cannot be complacent about what is consumed!

Here in the USA, the United States Environmental Protection Agency (EPA) has defined standards for water delivered by municipal water sources, therefore most public water supplies contain contaminates well below EPA dictated maximums. For instance, the EPA's standard for Nitrate (as N03-N) is a maximum of 10 ppm. But where more delicate invertebrates are concerned, levels even below this imposed maximum should raise concern. As for heavy metals such as copper, levels six times higher are federally allowed for drinking water than what would be safe for marine fish, and simply not tolerable at any level where invertebrates are concerned! In fact, if tap water is going to be used for the fish-only aquarium, it should contain less than 0.2 ppm copper. Nevertheless, keep in mind that some fish are sensitive to even smaller amounts of copper, e.g., angelfishes, butterflyfishes, and puffers to mention just a few.

Also keep in mind many homes or businesses have copper piping where stagnate water tends to accumulate a small amount of this element from the piping itself. Simply allowing tap water to run for a few minutes will flush the pipe clear of this element by bringing in a new supply of water that has not yet had time to accumulate copper ions. If the water from the tap needs to be processed before use, it can be flowed through activated carbon or an ion exchange molecular absorption filter pad, e.g., Poly-Filter™ until the copper level is within acceptable limits.

Where silica is concerned, excess can cause unwanted diatom algae and generally I've found it best if tap water contains less than 0.2 ppm. As for nitrate, any amount above a few ppm in tap water should be another signal to first process the water before use in the aquarium. When it comes to phosphate, which is a major plant nutrient, care needs to be taken to introduce the least possible, e.g., <0.02 ppm. Be aware some water suppliers periodically introduce a polyphosphate or a zinc orthophosphate compound into the water supply to reduce rust in old pipe systems. Hobbyists usually find out about these rust inhibitors when it's too late, i.e., when an algae bloom unexpectedly shows up in their aquarium. Recommend occasionally checking with water suppliers to find out if they are using these forms of water/system treatments.

Nevertheless, water coming from the tap is quite safe to drink; in fact it is more regulated than bottled drinking water! To be on the safe side where aquaria are concerned, it's recommended it be tested for copper, silica, nitrate, and phosphate, and if need be processed before it enters your aquarium. In some locales testing for iron may also be a good precaution. Keep in mind the water company customer in the United States is entitled to a free test of the water coming to their home or business per federal law. If not positive about its quality, discuss scheduling a test with the local supplier or at a minimum test it yourself.

If you deem further processing is required, most aquarists will use a home sized system to process tap water for their aquariums. The two most common systems use either deionization resins or reverse osmosis membranes. These systems can provide quality 'freshwater' for evaporation make-up or water changes. See Chapter 3 for a description of this equipment.

Last but not least, almost all municipal drinking water in the United States is treated with chlorine or chloramine (a combination of chlorine and ammonia) to kill disease carrying organisms. This also needs to be removed from the water before it's used in your aquarium and is discussed further on in this chapter.

Distilled Water

Some hobbyists have used distilled water in their aquaria thinking that if it is okay for the battery in their automobile it must be pure enough for their aquarium. That's not entirely correct! Even though distilled water is formed from condensation, the condensate, i.e., the water collected, could possibly contain trace amounts of copper if the equipment used in the distilling process is made from copper. Therefore, no matter what brand I use, its first tested, and if OK I'll stay with that brand.

Bottled Drinking Water

Available at most stores, especially grocery stores, bottled water should be considered a questionable water source for our aquaria since its not only expensive for the amount purchased, its basically unregulated quality-wise. Some may even contain small amounts of copper, as it may go through a distilling process as mentioned above. Therefore its consumer beware, as the water coming from a local municipal supplier to your tap is regulated by federal law, and even though it may need processing before going into our aquaria, direct use of bottled water presents an 'unknown' when it comes to its constituents! And even though I'm a consumer of various brands of bottled water, I prefer not to use it in my aquariums.

Well Water

Some folks still utilize well water in their homes. In fact, the number of wells for the purpose of decontaminating polluted groundwater supplies is growing in North America, whereas the number of wells for drinking water is fast diminishing. Presently, many drinking water wells are found near or in farm country where pesticides, nitrate and phosphate fertilizers have had the opportunity for many years to possibly percolate downward and reach the aquifer. In some areas of the United States, well water is no longer safe and farmers must depend upon surface water to irrigate their crops or for drinking.

Therefore, in my opinion, the wells that are still being used for drinking water should be checked yearly for contaminates. Even if they meet EPA drinking water regulations, the water should still be checked for plant nutrients, e.g., nitrate, phosphate, silica, and possibly iron before being used in the aquarium. And keep in mind that even though well water may meet all EPA standards, it is usually low in oxygen and almost always high in carbon dioxide. Therefore it should be aerated for 24 hours prior to use in the aquarium.

Water Softening

The use of softened tap water for either the fish-only or reef aquarium is not recommended simply because I've experienced letters spread quite evenly over the past couple of decades from those using so called soft water and all have experienced animal losses sooner or later. In fact, changing back to regular un-softened water cured their problems time after time. And yes, I'm not up to date on soft water processes in today's marketplace, but if thinking of going this direction, I highly recommend much research before its product water is utilized for the aquarium.


The use of ammonia as well as chlorine to disinfect drinking water has become a common practice here in the USA because it's a very stable compound and stays active throughout distribution systems. Not toxic to humans, but where aquariums are involved, extremely toxic to fish and corals. In fact, it is so stable that aerating the water will not remove it, as would it if it were only chlorine. Another reason for its use is that chlorine by itself produced a compound known as trihalomethane, a known carcinogen, so the trend has been to move away from solely using chlorine.

Therefore, for use in aquaria its removal is quite necessary! This can be accomplished by using an ample amount of sodium thiosulfate, which is normally sold as a dechlorinator, to the freshwater 'before' the salt mix is added. That will break the bonds between the chlorine and ammonia, negate the chlorine, and then with aeration allow the ammonia to escape as a gas. Always test with an ammonia test kit before using the treated water. Activated carbon can also be used to remove chloramine, however its a time consuming task and probably far less expensive to simply use a dechlorinator.

Natural Seawater

The processing of freshwater, if required, is only a small portion of the 'Water' big picture when it comes to our marine aquariums. In fact, the most important in whether we succeed or fail is the overall 'quality' of its seawater. This is especially true in reef systems where some corals and other invertebrates require high quality seawater for their continued wellbeing. Actually, that's not much different from the quality of air we breathe when you think about it! There's also no doubt the convenience of mixing a dry salt mix with freshwater to produce a suitable artificial seawater has been a boon to the marine hobby. But before discussing synthetic seawater, it's beneficial to have a broad understanding of the composition of Natural Seawater (NSW), which represents about 95% of all water on the planet Earth.

Note, because all of the following subject matter in this section is quite interrelated, there may seem to be some occasional repetitiveness, but it's there to be 'helpful' in better understanding the topics being discussed or what I like to call the overall 'big seawater picture.'

When it comes to the earth-wide cycling of water (otherwise known as the biogeochemical cycle), we think of it starting at the ocean's surface where it evaporates into the atmosphere. At sometime in its future, water vapor condenses and falls back to earth in the form of rain, snow, fog, etc., and the water eventually returns to the sea carrying various salts, which it has dissolved along its travels. It's now termed the 'Universal Solvent' for many easily understandable reasons!

The then constituents/elements (saltiness) in the seawater principally results from two sources: dissolved matter from landmass runoffs and that of mid-ocean volcanic activity. These elements are placed into three groups: Major, Minor, and Trace elements. Major elements include those with concentrations over 100 ppm, Minor elements have concentrations between 1 to 100 ppm, and Trace elements with concentrations of less than 1 ppm. See 'Measures' in the rear of this book for a comprehensive Table of Seawater Elements.

Of the 92 know naturally occurring elements, about 80 of them are found in seawater! These, along with other dissolved substances make up about 3.5% of its content, with the remaining being pure water. Actually, many of the 80 individual ions and other substances are of no importance to the long-term health of aquarium inhabitants.

Even though the amount of trace elements, organic compounds, and other substances such as life itself can vary in NSW, the ions of the eleven major and minor inorganic elements remain in constant proportion to each other regardless of the total amount of other dissolved substances. These are called the 'Conservative' elements, and they make up 99.9% of the 'dissolved matter' in seawater.

For example, the percentages of sodium and chloride ions in NSW are always in the same proportion to each other whether that seawater is concentrated or diluted. In fact, one could measure either or any of the other 'conservative' elements and this would provide the concentrations of all the others. This rule of constant proportions, also known as Marcet's principal (Chlorinity) was discovered in 1819 by Alexander Marcet. Those elements consumed in chemical and biological processes are called 'Nonconservative' elements, such as nitrogen, oxygen, carbon, and phosphorus.

Keep in mind the ions of different salts are constantly binding and unbinding with other elements, such as with the positive ions (cations) of sodium and the negative ions (anions) of chlorine combining to create the compound called 'sodium chloride' (NaCl). In fact, when all the pure water evaporates from a container of seawater, the two most abundant ions, i.e., sodium and chloride, are visible as a solid form of crystals – sometimes referred to as 'salt creep' by aquarists. As an interesting side note, if all the salt in the oceans were piled on top of the USA it would be slightly higher than the height of the average person.

It's also possible to segment seawater into four separate categories, i.e., pure water, inorganic matter, organic matter and life itself. As mentioned above, 'pure' is a relative term, as 100% pure water has yet to be created, even by man! Nevertheless it can be considered as what evaporates from the oceans and seas and returns as rain/run-offs from land masses. The inorganic matter consists of those major, minor and trace elements along with gases that are dissolved in seawater. Then there are the organic wastes from manmade sources, such as human and animal waste, pesticides, garbage and oil seeping naturally or not into the water. Life itself fits nicely into a segment by itself, as everything from huge to microscopic creatures abound.

Even though there is an ever-changing input into this complex solution whether in closed system or the wild, that in the wild has had the capability to remain stable for millions of years, differing from that in the closed system. Therefore one must keep in mind the animals we wish to maintain in our marine aquariums have existed in a solution that has been very stable for eons, and if those surrounding waters in our aquariums were to become different in some element/compound concentrations, it may possibly cause undue stress to these animals, or even death! And even though there is a difference between that of NSW and that produced by synthetic salt mix produces, the synthetic mixes found in today's marketplace will mostly suffice nicely to get a marine aquarium up and running. Notice I said up and running, as on-going maintenance of that solution will require much knowledge as to what's needed to keep it from becoming detrimental to the system and the animals it contains. Therefore, some thought must be given to certain elements/compounds that can differ in 'closed' systems so as not to unduly distort their concentrations.

Using Natural Seawater

For those living near a supply of NSW and having the means to collect it, how lucky they are! Of course, it's always wise to collect away from inshore areas, as that is where sewage, fertilizer, insecticides, heavy metals and other pollutants tend to collect. Once collected it can be directly used in the aquarium if utilized within about 12 hours of collection. If not, precautions need be taken because its plankton will then begin to die, and it will become polluted. Keep in mind even though looking crystal clear when collected it teems with microscopic organisms called 'plankton,' which are tiny animal and plant wanderers. When their food supply is consumed, usually each other, the resultant seawater can then become toxic.

If not usable quickly, storage in darkly located (prevents unwanted algae growth) sealed plastic or glass containers is feasible for up to two weeks without any treatment. But if knowing it will go longer than that, suggest adding one bottle cap full of regular household bleach to 50 gallons of the 'freshly' collected seawater and then aerating it for two days. After two days of aeration, turn off the air pump and let the water settle for a day, then vacuum/remove any sediment at the bottom of the container. If a smell of bleach remains, aerate again until no smell of bleach remains, then store and use as needed.

If there are any negatives with using NSW that one collects themselves, the possibility of introducing a disease, parasite or a pollutant are some possibilities. Furthermore, it can exhaust its buffering capacity somewhat quickly since it is not slightly modified, as is many dry salt mixes to supplement various chemical aspects in the closed system.

Seawater's Dissolved Components

There has been much progress over the past few decades in identifying specific elements along with their concentration that are required by fishes, invertebrates, algae, and/or algae/plant life in our seas. Those quantities may be large or small depending upon the life form, and most often come from the surrounding water, or the food consumed. And in caring for these life forms, it's beneficial to understand which are more important than others. Many, as it will become clear, are not testable with average aquarium water test kits, nevertheless, they deserve to be mentioned here since some have interesting aspects and are sometimes found in aquarium additives, e.g., 'trace' additives.

To help focus on these aspects of water quality, have divided these elements into three separate areas, i.e., Major; Minor; and, Trace. As mentioned above, an extensive listing of seawater elements can be found in 'Measures' in Section 6 of this book. Furthermore, those of significant importance will be singled out, such as those affecting alkalinity and pH, and discussed in detail in the following two chapters.

Major Elements (Over 100 ppm)

Calcium (Ca)

This is an important element for fish, as well as stony corals, some soft corals, organisms that secrete a calcium carbonate skeleton or use it for structure support and/or a protective shell. Those marine organisms are in some ways no different than human beings. Whether the calcium is for our teeth and bones or the shells and skeletons of marine organisms it can be considered one of the main elements in the 'mortar' of life.

The level of calcium in NSW is approximately 400 ppm and the use of this element in bone-like structures, plant structures, seawater buffering or chemical precipitations causes its dissolved quantity to almost always be on the downward path in marine aquaria. To keep up with its usage almost all aquarists, especially reef aquarists, find it necessary to add calcium in one form or another on a regular basis. See Chapter 11 for further discussion on this important element.

Chloride (Cl)

This is an essential element to the wellbeing of all plant and animal life, and is found in seawater combined with sodium, e.g., sodium chloride, which makes up 98% of the dissolved solids in seawater. Water changes should adequately maintain all biological functions associated with this element.

Magnesium (Mg)

It should suffice to say here that this element is extremely important, as its essential for plants and animals, and also a major player in the chemistry of seawater, especially its buffering capacity. In fish-only systems, waters changes should adequately supply its needs, yet not so in reef systems, where demands for certain elements are far greater. See 'Seawater Maintenance' for further discussion on this important element in the following chapter.

Potassium (K)

Has some biological importance and is mainly responsible for most of the osmotic pressure in intracellular fluids in both vertebrates and invertebrates. Algae also require it, and it has recently become a manageable substance, as there are test kits and additives to monitor/supplement it. Water changes along with some trace element additives could also possibly adequately maintain it.

Sodium (Na)

This element is present in seawater as 'sodium chloride' and is the main cation in extracellular fluids and is important in nerve functions. Water changes should adequately maintain all biological functions associated with this element.

Sulfur (S)

Essential to all life, and even though an important component in proteins, body fluids, fats, and skeletons, it's not a manageable element in marine aquariums. Yet, keep in mind its often found in 'trace element' solutions, but because of the normally high content in seawater as the compound 'sulfate' these solutions do not overly affect its concentration in aquaria.

Minor Elements (1-100 ppm)

Boron (B)

This element is of importance to the buffering capacity of marine aquariums, and usually enters as a component of the synthetic sea salt being utilized, various alkalinity supplements, dissolution of carbonate substrates, calcium reactors, and in foods. In the wild it provides about 23% of the buffer system, with the bicarbonates and carbonates providing the remaining 77%. In aquaria, pH dictates its importance and a declining pH could require a larger percentage of Boron to adequately buffer the system. Unfortunately the demands of calcification, algae photosynthesis, and even water changes with a sea salt deficient in boron will deplete this element. A possible signal indicating this element may be low would be large swings in pH, e.g., 7.8 in the early morning, and 8.3 - 8.4 during a lighted timeframe.

Testing for Boron is possible, as there are aquarium test kits for this element, e.g., Salifert Boron Test Kit. Generally, good aquarium husbandry and adequate water changes with a sea salt having an elevated amount of this element will suffice to properly maintain it at about 23% of the alkalinity system, or 4.6 ppm of the volume of the water tested. But care needs to be excised if supplementing, as excesses can be very toxic! The common grocery store product Borax can be used as a supplement, and one level teaspoon will increase its level in 100 gallons by 2.3 ppm. Keep in mind a range of 4 – 15 ppm is acceptable, with anything beyond that negatively affecting an 'alkalinity' test result. As a plant nutrient, Boron is only essential to the marine diatom Cylindrotheca fusiformis, and simulates growth in the macroalgae Ulva, and the brown macroalgae Dictyota. Whether or not it is important to Zooxanthellae, is still not clear.

Bromide (Br)

Has some biological importance, and appears to be an essential element for red algae. Brominated compounds are also found in bacteria, brown algae, coralline algae, anemones, sponges, and black corals. In some situations, it's used as an ingredient to foster the chemical defense system utilized by some invertebrate, such as associated with some sponges.

Bear in mind that ozone reacts with bromide and also chloride ions in seawater and results in a free-radical known as hypobromus acid, which can cause the death of some marine organisms. In fact, these strong oxidizing compounds can cause DNA damage at a level more than what 'chlorinated' compounds can cause! Therefore, some salt mixes limit the use of bromide in their mix so as to reduce this possibility. In fact, if the aquarium utilizes ozone, it may be wise to choose a salt brand with low levels of this element.

Nevertheless some marine organisms, especially certain species of algae and sponges seem to benefit from careful use of additives containing bromide. After using a Bromide/Fluoride additive in my aquarium for two months, suddenly had some very interesting sponge growths. In three areas bright yellow sponges, Leucetta sp., containing an erect central outflow siphon began to grow. In another area, a brown encrusting sponge began with several erect outflow siphons. This is not to say the additive was responsible, but the timing of the happening and the fact nothing new had been added to the aquarium in almost a year, does make me wonder about it!

Bromide can also be removed from the aquarium by protein skimming and activated carbon. Since there are no aquarium test kits for bromide and there are Bromide/Fluoride additives on the market, caution must be exercised in their usage. In fact, have discontinued its usage since there are too many unknowns. Periodic water changes should suffice to adequately supplement the needs of various aquarium organisms.

Fluoride (F)

As with human teeth, this element is an important part of strong teeth for fish and some invertebrates, and also plays a roll in the mineralization process and some other biological processes. In fact, its known that certain fish, e.g., tangs, triggerfish, parrotfish, and sharks have teeth coated with a fluorapatite (a mineral containing fluoride, calcium, and phosphate) that gives their teeth the hardness needed to crush less hard compounds. Nevertheless excesses are hazardous to marine organisms and any additives specifically designed to boost its presence in aquariums must be given careful consideration before use. To compound that situation, there are no aquarium fluoride test kits. Fluoride can be removed by protein skimming. See Bromide above for additional comments.

Thought should also be given to the direct use of tap water, as this element has been purposely added to various municipal water supplies for the purpose of enhancing the hardness of human teeth. Therefore the use of unprocessed water for water changes and/or evaporation makeup may be of a concern. Otherwise, periodic water changes should be adequate to maintain this element.

Strontium (Sr)

This element is found in marine plants and the skeletons of stony coral and varies in concentration from species to species, yet has no biological functions. It is chemically similar to calcium and many corals can substitute strontium for calcium and form strontianite instead of an aragonite skeleton. (Spotte, 1979) This may be due to the fact certain amino acids found in the organic matrix are believed to attract cations such as strontium. Strontium is also believed to inhibit tissue recession. Yet, other studies have shown strontium to inhibit the transport of calcium across the coral membrane. This could slow down calcification and inhibit calcium uptake for other metabolic functions. See 'Seawater Maintenance' for further discussion on this important element in the following chapter.

Trace Elements (Under 1 ppm)

As the name implies these are 'trace' elements, i.e., extremely small in quantity and that not enough is known about how many of these elements are used by the animals and plants in our aquariums. Nor are we sure of why some elements accumulate in aquarium water, although it is suspected inadequate protein skimming, lack of sufficient water changes, inadequate use of activated carbon, use of unprocessed water, foods fed and/or the brand salt mix used to be possible causes. Therefore care needs to be taken in the use of commercial trace element additives. To complicate the matter somewhat further, most trace element additives do not depict or accurately depict the actual quantity of its individual components. In fact, the components listed on the label may be more a good guess than from a scientific analysis. So stay with quality brands!

To make up for any depletion of valuable trace elements, hobbyists mostly depend upon water changes, yet some supplement their systems with 'trace additives.' Therefore, taking into consideration that water changes do add some trace elements my rule of thumb for trace element additives is always to begin by adding only one-quarter of the manufacturers recommended dose. In fact, many hobbyists usually find the quarter dose routine to be adequate. Over-dosing can lead to unwanted algae problems and once that begins, that's difficult to overcome.

If you decide to start with my quarter dose method, suggest waiting three months before increasing the dosage rate to half the manufacturer's recommendation if the system continues to look good. Should the aquarium continue to be free of unwanted algae for another three months, proceed to a three-quarter dose. If another three months go by without a problem, increase to a full dose. But don't exceed that thinking more is better. In my opinion, it's not!

The following are only some of the more widely researched trace elements and are presented here so one can judge their importance.

Aluminum (Al)

Has some biological importance, and has been found concentrated in species such as Caulerpa, Xenia, and Gonipora. It can enter the aquarium in several ways, e.g., from calcium oxide or calcium hydroxide or other forms of limestone medium used in making Lime Water as they may contain Al as a trace element. Aluminum based phosphate removers are also a source. It is also found in some food items, and possibly some trace element additives; however many of these additives are no longer allowing this element in their products. According to Homes-Farley (2003) there's lethal toxicity in a crab at 10. ppm, other crustaceans at 0.24 to 10 ppm, a mollusk at 2.4 ppm, and various annelids at 0.1 to 0.4 ppm. Al can accumulate in tissues and bind with phosphate, and therefore become part of hard tissues such as bones, which may rob other tissues of calcium and phosphorus. It may also cause fatty degeneration of liver and kidney tissue. Not testable with average aquarium test kits, but sensible and periodic water changes and the use of protein skimming should prevent any detrimental accumulations of this element.

Antimony (Sb)

It is said to stimulate metabolism, yet remains a Trace element of little or no special interest as water changes and food entering the aquarium can supply any minor needs.

Arsenic (As)

Where marine species are involved, only those of red algae seem to benefit from this element, therefore it's of no special interest to most marine aquarists. Furthermore, it tends to bind with iron minerals and be precipitated out of solution. Water changes should adequately supply any needs for marine animals, which remain mostly unknown at present.

Barium (Ba)

Barium carbonate appears to be a transition carbonate between that of calcium carbonate and strontium carbonate, therefore its found in some coral skeletons. Nevertheless its roll in marine organism metabolism is not fully understood. It remains an element of little or no special interest as water changes and trace element solutions can supply any minor needs.

Chromium (Cr)

Considered an essential element for certain biological processes, yet not one that needs the attention of the average aquarist as water changes and the food entering the system, along with some use of trace element additives should adequately maintain its presence.

Cobalt (Co)

Serves to enhance the utilization of Vitamin B12 and appears to be an important aspect in the coral skeleton, which obtains it from the food consumed. Requirements for this element can be adequately met with water changes and the food entering the aquarium system.

Copper (Cu)

Found in natural seawater at very low levels and is considered an essential element to all life. Yet in aquaria should it only slightly exceed that of what is found in the wild its toxic to most invertebrates. And if it exceeds 0.2 ppm, it becomes toxic to fish and plants. Usually not found in any salt mixes, however may be a very minor component of some trace element additives. For more information about using copper 'treatments' for various maladies, see Chapter 14.

Iodine (I)

This is an important element in many biological processes and also important to forms of red and brown algae. Deficiencies, e.g., can cause molting problems in some crustaceans, enlarged thyroid glands (goiters) in sharks, and reduced activity levels for some invertebrates. Various properties of the element are reported to serve critical roles in coral metabolism and integrated coral growth functions. Because of this and its ability to enhance redox potential, there has been many forms of additives coming to market of which many have been over utilized or incorrectly applied. Since it can kill both beneficial and harmful bacteria if used incorrectly, it is wise to understand much more about this essential trace element prior to increasing its presence in aquaria with additives designed specifically for that purpose. See 'Seawater Maintenance' in the following chapter for further discussion on this important element.

Iron (Fe)

This is another element having several important biological aspects and can be considered essential for marine plants/algae and especially animals that utilize the photosynthesis process to obtain some of their nutrition needs. In fact, it provides for oxygen transportation in the blood of invertebrates, and is vital in the metabolism of 'B' vitamins. Fish obtain it from the food they eat; however algae and animals that contain zooxanthellae need bioavailable iron. Deficient amounts of iron may limit the growth of photosynthetic organisms, yet mostly that of macroalgae such as Caulerpa. See Chapter 12 for more discussion on this element.

Lithium (Li)

No known biological functions, yet appears to be essential as deficiencies appear to cause some growth and function defects. Water changes along with the food fed should adequately control the level of this trace element.

Manganese (Mn)

This element is essential is several ways, e.g., enzyme activity, photosynthesis and growth of plants, algae, and zooxanthellae, and may be also important to the reduction of nitrite. Nevertheless, its not an element of concern in marine aquariums, as water changes and feeding should adequately supply/control this element.

Mercury (Hg)

There is no known biological function for this element, yet caution is advised, as this is a cumulative poison in that it accumulates in animal tissue. Therefore it's wise not to use the old fashion mercury thermometers in aquaria that are easily broken. Water changes along with the food fed should adequately control the level of this element.

Molybdenum (Mo)

Pronounced ma-lib-de-num, and in conjunction with phosphate and sufficient light, this element is an algae enhancer. It is essential to nitrogen-fixing organisms, e.g., cyanobacteria (red slime algae) and possibly zooxanthellae to some degree. Generally found as disodium molydate in aquarium additives, however, specifically adding this element is not recommended, as excessive levels are quite toxic. Water changes should suffice in regulating this element. Discussed again in Chapter 12

Phosphorus (P)

This element is an important constituent of DNA and RNA, and as a component of other molecules such as phosphate (PO4), which is a compound that requires close attention especially in reef aquariums, as it is an important algae nutrient. See Chapter 12 for a discussion on phosphate.

Silicon (Si)

The element Silicon is a metallic element that is second only in abundance on earth to oxygen. In fact, over 25% of the earths crust is made up of silicon in one form or another, but is not an element found freely existing in nature as it is found as silica oxide (sand and quartz); as silicates in rocks (granite); and dissolved in water as silicic acid, but by far the most abundant are oxides of silicon known as silicates. Of these, silica, quartz, jasper, and agate are examples. Varied forms of silicon oxides are also found incorporated into minerals such as mica and feldspar. Given the enormous existence of silicon/silicate rich rocks in the earth's crust, vast amounts of this element have washed into the oceans where it is found dissolved as Silicic acid (H2SiO3).

Even though silica, as silicate varies greatly in NSW, e.g., .04 - 8+, anything over 0.2 ppm in closed systems may cause a diatom bloom. Silicate is usually introduced into the aquarium via tap water, a salt mix, silica sand dissolving slowly (possibly in fluidized bed filters), or something in the aquarium that may be composed of a silica substance. See Chapter 12 for more discussion on this element.

Sometimes excluded from lists of elements due to its non-conservative nature/nutrient nature and inconsistent levels in NSW. As silicon dioxide, it is essential to some marine life, e.g., diatoms, some protozoa and sponges.

Vanadium (V)

An essential Trace element to many organisms, especially tunicates that seem to have amounts a million times higher than what's normally found in seawater. It's also found in concentrated amounts in single cell algae and plankton. Nevertheless, all requirements in marine aquarium can normally be met with water changes, or additives containing this element.

Zinc (Zn)

An essential element to all plants and animals, yet not a manageable element and water changes will provide adequate replenishing of this element. Usually found in most trace element additives.

There are of course many other trace elements, e.g., Cadmium (Cd), Cesium (Cs), Beryllium (Be), Bismuth (Bi), Germanium (Ge), Lead (Pb), Nickel (Ni), Radium (Ra), Rubidium (Rb), Scandium (Sc), Silver (Ag), Thallium (Tl), Tin (Sn), Tungsten(W), and Uranium (U) to mention a few. Most have no known biological importance and are not elements of concern, unless they become concentrated in aquaria, thus becoming potentially toxic to the life in the system. Water changes, along with proper filtration methods should adequately control the level of these and other nonessential trace elements.


Some of the elements found in seawater are present in the form of dissolved gasses. Those considered as conservative are the biological inert gasses, e.g., helium, argon, neon, and krypton. Their concentration depends on their equilibrium values between the atmosphere and seawater. The non-conservative gasses include oxygen, carbon dioxide, nitrogen, carbon monoxide, methane, nitrous oxide, hydrogen sulfide, and hydrogen, which are determined by biological and chemical processes and which are not typically included in a list of elements due to their varying gaseous states. Only four of these are important to marine aquarists, i.e., oxygen, carbon dioxide, nitrogen, and hydrogen sulfide. A fifth, methane, is also a possibility now that mud-like refugia have gained some popularity.

Oxygen (O2)

Almost all life requires oxygen, a simple molecule containing two atoms of oxygen. And the air we consume contains about 21% oxygen, which is not only important to us, but also the animals in our aquaria because when combined with hydrogen, its known as 'water.' Yet, even though 21% of the air around us is composed of oxygen there's only about 6 - 7 ppm in the water that surrounds our submerged aquarium animals. Since there are many thousands of times less oxygen in water than in air that should be the first clue that oxygen levels in the aquarium should be closely monitored.

Another interesting point is that in man or fish the hemoglobin (blood) flowing through their vessels collects oxygen from either the lungs of man or the gills of fish and delivers it to various parts of the body where it then collects carbon dioxide and carries it back to the lungs or gills for exchange with life giving oxygen. Should there be too little oxygen in solution the possibility exists the hemoglobin could transport carbon dioxide in both directions. Depending upon the percentage of each in solution that situation can be anything from slightly stressful to the expiration of life.

Seawater can only hold a relatively small amount of oxygen as mentioned above, and when it reaches the maximum it can hold under normal circumstances it is called 'saturated.' Nevertheless there are some special conditions where it will briefly contain a greater amount and that is referred to as 'supersaturation.' Waves breaking over exposed surfaces will surround such areas with sometimes amounts of dissolved oxygen reaching almost 9.0 ppm! As for the norm in the wild, especially around some coral reefs, the amount of dissolved oxygen during daylight hours is approximately 6 - 8 ppm, and in nighttime hours about 5 - 7 ppm.

Therefore, to maintain marine aquatic animals in the best condition possible the aquarist should always strive towards maintaining dissolved oxygen at or near saturation for the system's temperature, e.g., dissolved oxygen at saturation in a cold water aquarium where temperature is maintained at 59°F is approximately 8.1 ppm. Yet, at the temperature of 77°F, at which many reef aquariums are maintained, oxygen saturation is much lower - near 6.8 - 7.2 ppm. And keep in mind most fish suffer severe stress if it drops below 3.5 ppm and die when it drops to 2.5 ppm (Moe, 2009).

Fortunately for the fish keeper a correctly engineered and maintained trickle filter is a very good way to boost bulk water oxygen levels. However the same is not true with undergravel filters, which remove oxygen from the water and generally operate at levels of about 5 ppm, as some of my far past systems have proven. Fluidbed filters are also in the same category as UGF's. And unfortunately at this level some animals begin to experience mild stress and at lower levels begin to exhibit stunted growth and/or poor digestion. It's a downward slide from there!

As for what is the minimum acceptable oxygen level in aquarium systems, consider that to be a maximum of 1 - 2 ppm below whatever is saturation at its present temperature and specific gravity. Below that, animal stress becomes a very real possibility; with it becoming lethal as it reaches 2 – 3 ppm. Oxygen test kits are fairly inexpensive, and all type systems should be checked at least once per month, with one test at the middle of the lighted timeframe and a second test early in the morning before system lights turn on. If either is alarmingly poor, recheck the procedure for doing the test and try another sample. Maybe even try a second brand test kit. A great disparity in a reading is cause for major evaluation of the bioload and equipment in use.

Keep in mind that one of the best areas in the aquarium to gain oxygen is at its surface; therefore do not completely cover the aquarium with glass or plastic covers, as that simply reduces the natural gas exchange at the water's surface. Even without covers, aquarists who do not use a protein skimmer or surface overflows are somewhat in the same predicament as if a cover were being used! In fact, the use of a simple inexpensive surface overflow will consistently remove the thin film of organic contaminants that gather on water surfaces and interfere with good gas exchange. In some cases, especially large systems, surface fans that attached to the upper edge of the aquarium are available and are excellent for gas exchange and if need be, can also provide some cooling benefits.

Protein skimmers, besides aerating the water also help to remove some of the organic matter that would have required increased biological oxygen demand (BOD). Airstones and/or powerheads located at different levels in the aquarium will also help produce good circulation and aid gas exchange. Also, keeping substrate and mechanical filter media clean will help reduce demand on dissolved oxygen (BOD). The use of quality lighting combined with macroalgae will also help in some instances to remove some pollutants and add oxygen to the bulk water if properly maintained during lighted timeframes. And, there is always overcrowding and/or overfeeding to consider because it strains the systems ability to keep oxygen adequately replenished.

Quite a few years ago I thought owning an oxygen test kit was not necessary. But after seeing the high prices associated with corals and fishes now consider their comparatively small cost a sensible addition to my array of water test kits. Bottom line, its a small price to pay when you have a large investment in the aquarium. In knowing the level of this important element, both you and your aquatic inhabitants should breathe a lot easier!

Carbon dioxide (CO2)

Even though oxygen makes up about 21 percent of the air we breathe, our aquatic animals live in an environment that contains much less. In fact, it contains over 20,000 times less available oxygen than what is in the air we breathe! That's because oxygen is only slightly soluble in water. Unfortunately the same is not quite true for carbon dioxide. Even though it's a very minor constituent of the atmosphere, about 0.03%, it is more soluble than oxygen.

As noted above, the atmosphere contains much less carbon dioxide than oxygen, 'and' what is normally found in water is also less than the amount of oxygen in the blood of its animals. Therefore, when dissolved oxygen and carbon dioxide in solution are in equilibrium with the atmosphere, the animals hemoglobin will carry oxygen to various portions of the body and transport the waste, i.e., carbon dioxide, back to the gills for disposal into the surrounding water as noted in the oxygen discussion above. However, should there be poor gas exchange in the system there is the very real possibility 'accumulating' carbon dioxide will displace some of the oxygen in the animal's bloodstream, which creates a stressful condition. In severe cases there's the possibility the hemoglobin could transport carbon dioxide in both directions. Doom and gloom for both the animal and the hobbyist. Besides animal health problems, excessive carbon dioxide will also lower pH and for those with reef aquariums, make calcification processes more difficult.

Keep in mind as free (gaseous) inorganic carbon dioxide from the atmosphere enters seawater it is rapidity converted to carbonic acid, then becomes part of the alkalinity buffering system where it shifts back and forth in equilibrium reactions tending to maintain pH at 8.1 - 8.2. Dissolved inorganic carbon dioxide also enters in rainwater and run-offs from rivers and via biological processes where it is a waste product of metabolism and plant respiration. Bacteria release organic carbon dioxide when they process/metabolize detritus, uneaten food, dead animals or plants.

In closed systems the trend is usually towards carbon dioxide accumulation since it does not leave solution easily. Conditions such as tight fitting aquarium covers, poor aeration, overfeeding, overcrowding, poor mechanical filter media maintenance, inadequate/no protein skimming, poor lighting, inadequate water movement, excessive detritus build up, excessive carbon dioxide from calcium reactors, or improperly set carbon dioxide injection equipment, amplify its accumulation. But don't panic and throw fifty aerators in the aquarium hoping to rid the aquarium of all carbon dioxide since it's a source of carbon and all living things require it. Furthermore carbon dioxide is essential for photosynthesis and the formation of calcium carbonate, e.g., the structural material of reef building corals.

Nevertheless its level should be given some forethought. In fact, larger aquariums may be more prone to carbon dioxide buildup than small aquariums. The more volume, generally the less water surface area there is in relation to the system's volume. In other words, you may have six clothes closets all with 3-foot wide doors to enter. But their interior space can vary. Since it's the aquarium's volume in relation to surface exchange area that impacts good gas exchange, larger aquariums need a little better planning when it comes to water movement. Therefore they should have more top to bottom circulation than horizontal water movement. This helps agitate the surface and allows oxygen to travel more freely into the aquarium and excess carbon dioxide to exit. Even a circulation fan blowing on the water surface is a great help, as is surface skimming.

On the other hand, dissolved carbon dioxide can quickly be exhausted when plants and algae photosynthesize under proper illumination, then turn to carbonates for fuel to continue the photosynthesis process. This precipitation/reduction of carbonates causes pH to rise. Then during unlit timeframes respiration from plants and animals add carbon dioxide back to the water, driving pH lower. Of course, reverse lit refugia containing a somewhat more than modest level of algae can help alleviate this condition. And if the hobbyist wishes to maintain a system where macroalgae is the main theme, then a minimum free carbon dioxide level of 3 - 5 ppm should be maintained, possible using timed injection of carbon dioxide to correspond with only the well-lit timeframe. Otherwise, in a well-maintained aquarium anything over 2 ppm should be considered time to review one's system husbandry.

Overall, most hobbyists do not test for this gas unless the system is experiencing a declining pH, e.g., <8.0, as some think by using a pH buffer their problem is resolved – not true! At this stage, the proper thing to do is first test alkalinity and if low, bring it into a 'balanced state' with calcium (see Chapter 11 for those details). To test for excess carbon dioxide, simply take a small amount of aquarium water and aerate for twelve hours. Then test both the aerated water and that of the aquarium with a pH test kit. If the sample has a lower pH by more than .2 units than the aquarium bulk water, the aquarium contains excess carbon dioxide and improved equipment or maintenance is in order.

Periodically using a carbon dioxide test kit and keeping a record of the system's alkalinity and calcium, along with pH and possibly dissolved oxygen readings taken in the early morning and lighted timeframes would serve to educate in the interplay between these parameters over the long run. Then, the hobbyist could possibly recognize trends needing attention before serious problems arise.

Hydrogen sulfide (H2S)

This gas is formed by the activity of bacteria on organic matter in environments experiencing little or no oxygen. Its actually part of the sulfur cycle, a never-ending cycle both in nature and the aquarium. It moves from sulfate to sulfide, then back to sulfate with the help of various bacteria. In areas where there is insufficient oxygen to return sulfides back to sulfate, hydrogen sulfide continues to accumulate.

In nature, the deeper you go the less oxygen there is in the substrate and pockets of hydrogen sulfide are commonly found in deep areas. Yet, in aquaria it should not be found and even more importantly, not disturbed if found because if released into the bulk water it's highly toxic to most animals. In aquaria containing deep fine-grained beds, or those with its bed surface blocked with excessive rock, hobbyists often see dark areas in their sandbeds, no doubt indicating the presence of dangerous hydrogen sulfide gas. This gas has a rotten egg odor, yet if seen but not smelled, small darken areas may be of no concern and should not be disturbed, as that would allow its entrance into the bulk water. Without any intrusion into those dark areas, it 'may' stay isolated, thereby not a threat to the systems animals. But it's an indicator that anaerobic activity is generating hydrogen sulfide and there is insufficient oxygen in the bed to complete the entire sulfur cycle. It would then be wise to review system bioload and the techniques needed for improving biological filtration.

Sulfur Cycle

Dissimilatory Sulfate Reducing Bacteria (SRB), using excretion and decomposition produce sulfide. The sulfide can reoxidize back to sulfate by the actions of Colorless Sulfur Bacteria (CRB) and Purple Sulfur Bacteria (PSB). The activity of aerobic heterotrophic organisms in upper sand levels depletes oxygen, and fermentative organisms in this rich area of organics provide growth substances for the SRB. Whether in the Jaubert sandbed or other forms of sandbeds, the process is the same, the SRB reduce the sulfate to sulfide. In fact, the SRB move upwards in the sand column at night to get more oxygen since the sandbeds oxygen level in general decreases at night. In fact, this is good reason to have adequate water movement in aquariums containing sandbeds since without it, some areas near the substrate surface that experience very little or no water movement may experience fish deaths because they chose those areas to rest at night. As for sulfide, in micro-areas where there is no oxygen, PSB will oxidize sulfide to sulfate provided there is light. Where there is useable oxygen, almost all sulfide will be oxidized back to harmless sulfate by CSB. Actually, CSB has higher affinities for sulfide than PSB.

Nitrogen (N2)

This is one of the most abundant elements on Earth, as it makes up about 80% of the atmosphere. Generally nitrogen gas is at equilibrium with atmospheric pressure both in NSW and in aquariums, and usually remains at about 10 - 15 ppm. As long as oxygen is present, biological processes such as denitrification that result in free nitrogen gas do not affect its level in water because bacteria and blue-green algae use a process called 'nitrogen fixation' to reuse the liberated nitrogen to form various compounds; such as the nitrogen-laden compounds that concern aquarists, e.g., ammonia, ammonium, nitrite, and nitrate, therefore it remains a conservative element.

By itself, this not an element of concern where aquarists are concerned unless its level becomes 'supersaturated.' This could happen if there were an air leak on the inlet side of a high-pressure pump and its air and water is injected into the aquarium water, thus creating very tiny air bubbles in the bulk water. Of course the air in these tiny bubbles contains a very high level of nitrogen, therefore the fishes and invertebrates can ingest or absorb this nitrogen-laden air and develop the 'bends,' sometimes seen as 'pop-eye' or simply bubbles elsewhere on the body. If such conditions appear, the fix is to repair the pump as needed.

When existing in nitrogen-laden compounds such as ammonia (NH3), it is very toxic. As ammonium (NH4) it can, depending upon pH be somewhat toxic, and if as nitrite (NO2), again somewhat toxic depending upon its concentration. As for nitrate (NO3), it's relatively non-toxic. In aquaria, the biological denitrification process converts nitrate back to nitrite, dinitrogen. Import and export of nitrogen compounds in closed systems are an essential part of their maintenance.


Another toxic gas that forms from the activity of bacteria on organic matter, especially in swamp-like substrates is methane. The major difference between hydrogen sulfide and methane is that, unless disturbed, hydrogen sulfide stays sequestered whereas methane naturally bubbles upward to the surface of the substrate. In fact, if one were to ever walk through a swamp-like environment, bubbles of methane might be seen bubbling up to the surface. Furthermore, one can smell the rotten-egg odor of hydrogen sulfide, but methane is odorless.

The reason to bring this gas to attention here is because I had the opportunity via email to discuss a large reef system where all it inhabitants were found dead one morning by its owner. This large aquarium, about 400 gallons had a three-year-old large 6-foot long mud-based refugium attached for filtration purposes. Over several lengthy emails we went over all possibilities thoroughly, including naming all the livestock, their outward condition when found dead and oddities such as electrical shock, yet were not able to come a single cause. Then questions as to air pockets in the dark, deep (over 4 inches) mud substrate arose, and if a rotten egg smell was ever detected above the refugium, which it wasn't. I then asked the owner to disturbed the mud substrate in some areas to see if bubbles could be found and if a rotten egg smell was detected. Some were found; yet there was no odor, indicating they may have been methane and if so, could be the cause of his animal losses.

Furthermore, the aquarist torn down the entire refugium and noted in a following letter more pockets of 'air' were found, yet never detected a smell of rotten eggs. To this day I question the possibility that methane was the cause, simply because it's odd that some hydrogen sulfide was not also found in what was a densely crowded refugium filled with various forms of animal and algae life. Nevertheless, one should realize that methane and hydrogen sulfide are normally found in organic-loaded swamp-like substrates, with both quite deadly. But must add, I've not heard of one other similar happening, therefore, very possibly something else may have caused his loss and must give mud-systems a clear bill of health as to methane problems at this point in time.


Lets now move on to Seawater Management in Chapter 10, which I think you'll find quite interesting and informative.