By Bob Goemans
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Seawater Buffering

Authored by: Bob Goemans

In the wild there are always sufficient buffering elements and/or compounds to maintain pH at about 8.2. Nevertheless, in our aquaria those constituents, generally called ‘alkalinity,’ are constantly consumed, especially in reef aquaria where its ‘calcium’ content becomes a major player in the buffer system. Probably the most widely used method for maintaining its players; pH, alkalinity, and calcium, are with ‘Kalkwasser’ additions.

Kalkwasser (chalk water/calcareous water) also referred to as Limewater, can be utilized to makeup for evaporation and at the same time add calcium and control pH and alkalinity. Wilkins (1973) is credited with the development of method. Kalkwasser is most often prepared by dissolving calcium hydroxide (Ca(OH)2) in purified (reverse osmosis/deionized) water. This is a fine caustic powder, almost talc-like, and if inhaled or enters the eye, can cause severe tissue damage, and when dissolved, has a pH of 12. Calcium oxide (CaO), sometimes called Quick Lime is somewhat safer and less expensive to use, yet is lumpy and more difficult to dissolve. It also generates much heat as it dissolves; therefore caution needs to be taken as it could burn the skin if mishandled. In dry forms, both should remain sealed in closed containers until used, as they will absorb carbon dioxide from the air and become less useful/soluble. Both supply the same ‘balanced’ approach to maintaining calcium and alkalinity.

There are some benefits provided with the use of Limewater that other forms of calcium additives do not provide. The most important is the introduction of two hydroxide ions (OH-) for every one calcium ion. The hydroxide ions provide alkalinity by combining with dissolved carbon dioxide (CO2), producing bicarbonates and carbonates, and pushing pH higher, preferably maintained in the range of 8.2 – 8.4, which also helps to improve protein skimming efficiency. The added calcium, in the form of free ions, is immediately available to the carbonate/bicarbonate system and calcifying animals.

Overall, the addition of Limewater increases the demand for dissolved CO2, either from respiring organisms or that which can ‘slowly’ enter from the atmosphere when the bulk water tries to remain in equilibrium with the air above it since it is now containing somewhat less CO2 because of the addition of Limewater. In fact, the supply of CO2 may or may not be adequate in some systems, especially those aquariums with high calcification rates and/or low evaporation rates.

And because pH rises with the reduction of dissolved CO2, carbon dioxide injection equipment or increased evaporation rates may help alleviate these conditions, which are explained further on in this article. In addition, the high pH of the incoming Limewater fluid may precipitate some phosphate as calcium phosphate. Another plus is that when made with a high quality product, it does not contain unwanted salts such as sodium and chloride.

Bulk supplies of dry calcium hydroxide powder can be purchased in several different ways. It can be bought through chemical supply companies in a very pure ‘reagent’ grade or a less pure ‘technical’ grade. It can even be purchased at a grocery store as ‘Pickling Lime.’ The difference between them is simply the amount of impurities each contains. Of course, many aquarium product companies sell a high quality calcium hydroxide under their brand name.

Nevertheless, some hobbyists use only pickling lime, and since this is a human food additive and government regulated in many countries it appears to be suitable for aquarium use. Pickling lime is slightly greater than 95% pure calcium hydroxide. It also contains small amounts of magnesium oxide, calcium oxide, and trace amounts of strontium, sodium, and potassium. Some hobbyists report that aquarium water has a slight yellow color from its use, yet coral continue to do well and pH and calcium levels seem to be more stable.

Only about one ‘rounded’ teaspoon full of calcium hydroxide will dissolve in one gallon of purified water, producing a saturated solution of calcium, i.e., about 900 mg/l. Processed water is preferable so as to reduce the possibility of unwanted elements/compounds from entering the solution, e.g. phosphate, silica, nitrate, and iron. If more powder is added than what can dissolve, it will settle to the bottom of the container. Generally, a newly mixed solution is allowed to stand for an hour, and then the clear calcium saturated liquid is drawn off and used as soon as possible. Simply adding more water can dissolve any remaining calcium hydroxide powder at the bottom of the mixing container, however, this cannot be repeated too often because CO2 in air above the liquid reacts with the dissolved calcium, thereby precipitating an insoluble calcium carbonate.

Therefore the newly prepared Limewater solution should be stored in a tightly closed container and used as soon as feasible. No airstones should be used to stir the mixture, as that would only introduce more CO2 and form more precipitant. The mixing container should be cleaned at least once or twice a week to remove any precipitant. If cleaning does not occur often enough one may think the white power in the bottom of the mixing container was still calcium hydroxide, but not so!

Again, if the container is not cleaned often enough, an insoluble precipitate may be added to the aquarium thinking it is the calcium ion. This could very possibly lead to a false calcium reading. A possible indication that this was occurring would be a clear slimy gel-like substance on the mechanical filtration pads and/or difficulty in maintaining alkalinity.

Control Techniques

Because of its high pH, it is preferable that Limewater be dripped slowly into an area receiving a high flow of water. Large amounts added too quickly would swiftly raise system pH, possibly too high, e.g., >8.5, and shock it animals. To overcome this disadvantage, there are several ways to proceed to keep system pH within proper operating limits and still supply an adequate amount of calcium to meet most calcifying needs. There are then two aspects that primarily come into play, and they are the amount of evaporation a system experiences each day, and the amount of dissolved CO2 available to help reduce the overall impact of the incoming pH of the Limewater solution.

Manual Dosing

In systems with minor calcification needs, its possible to manually drip/pour some Limewater into an area receiving strong water flow to maintain a balanced calcium/alkalinity level. The key word there is ‘some,’ as the amount added in relation to system pH is quite a limiting factor as noted above. Also, if evaporated water is automatically replaced with freshwater, the amount of additional Limewater, especially in small systems, may cause an overflow. If manually applied, space each addition by a few hours; keep an eye on system pH, and the systems water level. Feasible, yet, labor intensive!

Bear in mind that if Limewater is ‘flowed’ into the aquarium there will be a large area where a too rapid increase in pH is occurring. This increased pH results in the loss of carbonate hardness (buffering capability) since calcium, magnesium and strontium are then very possibly precipitating out of solution. Yet, if prudently administered, dissolved CO2 in the system’s water may have the opportunity to adequately reduce its high pH. At a minimum, thoughtfully applied Limewater greatly reduces precipitation.

Drip/Evaporation Method

When the method first became popular, the clear Limewater solution was dripped into an area receiving swift water movement at a rate that would match system evaporation. Keep in mind when first introduced, the Limewater immediately reacts with any available dissolved CO2. If there is insufficient CO2, system pH can be driven dangerously high. And with most reef aquariums well aerated, there is usually little available to adequately keep pH from getting too high. In fact, insufficient CO2 is why clouds of minute calcium and carbonate crystals form when first introduced into aquarium water. Keep in mind these precipitating crystals go somewhere, usually on your sand grains/live rock or pump housings/impeller shafts.

To somewhat alleviate this situation; Limewater was dispensed during evening hours when systems experienced higher dissolved CO2 levels. Also, increasing aquarium surface agitation, with the use of fans, helped increase evaporation in some situations. Yet, depending upon calcification rates and system evaporation rates, the rate of drip, even on a 24/7 schedule, did not always supply system need to maintain its calcium and alkalinity needs.

CO2 injection

Along came the CO2 injection method, which is in some ways is no different that what is used in the freshwater hobby to encourage the growth of plants. Bottled CO2 gas along with a pH controller and solenoid valve would metered the input of CO2 into the aquarium water to maintain a desired pH level, usually in the range of 8.2 – 8.4. Therefore, the lack of sufficient CO2 in the bulk water to adequately compensate for the high pH of the incoming Limewater was resolved. And if evaporation rate and input of the clear Limewater matched system calcification needs, overall system calcium and alkalinity needs were met. Of course, the injected CO2 must not be near the area where the Limewater is injected, as that would result in the formation of calcium carbonate and both carbonate and calcium would then be lost before they can properly be used.

Milky-white method

Nevertheless, even with CO2 injection, some systems with very high calcification rates cannot supply enough calcium to sustained growth. Keep in mind the ‘clear’ Limewater contains relatively little calcium, especially when compared to other calcium supplements. To increase the amount of calcium a clear saturated solution of Limewater would deliver, it is placed into a closed mixing ‘reactor’ containing an additional amount of calcium hydroxide powder. In the bottom of this mixing vessel is a magnetic stirrer, which when activated, stirs its contents resulting in a milky-white solution containing undissolved calcium hydroxide powder. When this solution is delivered to the aquarium, the undissolved powder will dissolve in the aquarium water resulting in an additional amount of calcium being delivered to the system. Again, dissolved CO2 is a limiting factor, and this method is best used in conjunction with CO2 injection equipment.

Vinegar Additions

In what appears as a never-ending, yet welcomed thought trend on how to enhance or dispense Limewater into marine systems, there are thoughts on adding white vinegar (Bingman, 1999, 2000), or chelated calcium to the solution. The thought behind this is that the addition of an organic carbon, such as what is in acetic acid (white vinegar/5% acidity) in small amounts, i.e., 15 ml/gal, would provide additional carbon that would become a food source for bacteria in the aquarium, which oxidize this carbon and in turn generate inorganic CO2. The more dissolved CO2 in the system, if controlled properly, equates to the system being able to contain a greater amount of alkalinity and calcium, e.g. about 10% greater. In fact, it may also promote additional denitrification in the aquaria’s substrate, thereby reducing nitrate levels. The addition of white vinegar also neutralizes some of the Limewater’s high pH, reducing some of the concern about this aspect. Yet, one should keep in mind that the acidity of white vinegar varies, so care must be used if trying this modification, with the above amount suggested as the maximum to apply.

Of course, chelated products or calcium acetate could also be added to Limewater. However, there is concern the additional carbon source associated with these products can create unwanted algae growth. Nevertheless, reports have surfaced with the vinegar addition that have perceived additional coral growth, reduction of unwanted forms of algae, and increased skimmer efficiency. In addition, there is said to be some sand clumping, possibly caused by increased bacterium growths, as they were easily broken up. Calcified clumps, caused by calcium precipitation would have been more difficult to break up. Much care is advised if this technique is tried.

Gerbil Method

Another interesting yet somewhat tricky method to apply Limewater is with what can be called the Gerbil method, which is nothing more than an inverted bottle filled with clear Limewater and securely placed with its opening just touching the systems water level, preferably the sumps water level and an area receiving little or no surface ripples. When evaporation lowers system water level, gravity allows some of the bottles Limewater to enter the water to replace it. A two-liter soda bottle with a small hole in its cap, possibly a short length of rigid airline tubing cemented through its cap would suffice nicely. It should go without saying that much care needs to be taken with bottle support so its contents do not accidentally spill into the aquarium water!

Intravenous (IV) Bag

There are also medical intravenous (IV) bags that have a drip control valve that can be filled with Limewater and hung above the tank to slowly administer the solution. Gravity is simply the driving factor in getting the solution into the tank. Keep in mind they have very small dimension tubing, and need frequent checking, as calcium carbonate will sooner or later clog them. Replace the tubing as needed, or fill the bag with vinegar and allow it to flow through the tubing into a pail, then discard its contents and reuse the bag and tubing without flushing it with freshwater, as some remaining vinegar in the bag and tubing is not harmful.

In closing, even though there are many different aquarium goals, all consume its alkalinity base, i.e., carbonate and bicarbonate ions, because ‘life’ in its enclosure is producing waste compounds that create acids, which in turn need to be acted upon to keep pH from dropping to unacceptable levels. When properly used, Limewater is among the best ways to maintain alkalinity, pH, and calcium levels, as all these parameters are intertwined in seawater’s buffering chemistry.

Balanced Seawater Parameters

Alkalinity Calcium

2.0 meq/l - 5.6 dKH 400 mg/l

2.5 meq/l - 7.0 dKH 410 mg/l

3.0 meq/l - 8.4 dKH 420 mg/l

3.5 meq/l - 9.8 dKH 430 mg/l

4.0 meq/l - 11.2 dKH 440 mg/l

4.5 meq/l - 12.6 dKH 450 mg/l

(Source –

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