By Bob Goemans
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A Buyer's Guide To Calcium Additives

Authored by: Matt Clarke

Calcium is essential to the growth of coralline algae and most invertebrates, especially corals. It is taken up rapidly during growth and needs to be replenished regularly. It's just one of a group of water parameters, including alkalinity, magnesium and salinity, that need monitoring closely if you keep inverts, and it's important that the levels are maintained at a concentration similar to that of natural sea water. In the case of calcium, that's somewhere around the 400 - 420ppm mark. If the levels of these minerals drop too low, it may have a detrimental effect on coral growth and health.

In many cases, newly mixed salt water doesn't even come close to these levels and it's often necessary to replenish the mineral levels. The chemistry of newly mixed salt water varies according to the type of salt you use, and the mineral content of the freshwater you mix with it. Some salts on the market are designed for use with hard, alkaline tapwater and if they're used in conjunction with RO water, the water produced will be lacking in certain important chemicals, especially calcium and magnesium.

There are several salts on the market designed for use with RO water that contain additional minerals to compensate for the low mineral content of the water. In many cases, the salt water produced closely resembles sea water, and in some the calcium and magnesium levels exceed those found in salt water - something seen as beneficial by most modern reefkeepers, some of who now aim for calcium levels of 450 - 480ppm. We tested salt mixes in the September 2001 issue and found that Kent Marine, Instant Ocean and Reef Crystals produced the best water when mixed with RO.

Supply and demand

How much calcium you need to supply depends on the calcium demand of your system. If you've got lots of calcareous algae on your live rock, as well as clams and hard corals, demand is going to be fairly high. You can get a rough idea of how much is being used by measuring how much it drops each day or week.

In some large systems containing significant coral growth, demand could hit 30ppm per day, although it's usually a lot lower than that.

Calcareous substrates, like coral sand, crushed coral, cockle shells and dolomite aren't much good for boosting calcium levels, pH and alkalinity because they don't dissolve until the pH drops below 7.6. Aragonite is better, because it dissolves at pH 8.2, but it's often not enough to do the job on its own and is only really likely to help maintain stable levels.

For systems with low calcium demand, frequent water changes with a mineral-rich salt mix could be enough to maintain the levels seen in natural sea water.

If demand is higher, or you are aiming for calcium, alkalinity and magnesium levels beyond those seen in natural sea water, you will need to supplement these minerals. There are a number of ways to do this:


Calcium reactors are now widely regarded as the best option, because they'll control the calcium, magnesium and alkalinity and add trace elements in a balanced fashion. These units generally sit below the aquarium alongside the sump and need to be fed by a small water pump and supplied with carbon dioxide (CO2) gas from a pressurized bottle.

By passing water slowly through the aragonite-filled reactor, and using CO2 to lower the pH, it's possible to dissolve the aragonite more quickly - allowing it to release lots of minerals into the water. When running properly, these reactors give superb results. They are also much safer than some other forms of control and have fewer negative effects upon overall water chemistry.

Their only real drawback is financial - once you've bought the gas, regulator, hoses and reactor you're looking at a bill of at least a couple of hundred pounds, plus some running costs. However, as our calculations prove, it may actually be more sensible to invest in a reactor than to use additives if you have a large tank or a high calcium demand.


For many people, especially newcomers to reefkeeping, additives seem to be the easy option. They're often simple to add, and many are relatively cheap to buy for small systems.

There are dozens on the market, many of which are based around different concentrations of the same basic chemicals. If you can't afford to buy a reactor, they're probably the next best bet. However, many of them can cause imbalances in the chemistry of your water with long-term use. Some produce excess sodium or chloride which can affect salinity, for example, and it's easy to throw alkalinity and calcium levels out of balance with some additives.

There are several different groups of supplement, each based on slightly different chemicals, and each giving a slightly different overall chemical change. It's not always clear what you're getting though. Some manufacturers don't let on exactly what's in their products, or tell you the concentration they provide, or the sort of increase in calcium you can expect from dosing a specific amount...

The additives on the market for replenishing and maintaining calcium levels fall into three main categories:


Kalkwasser, or limewater, is a highly caustic liquid additive made by adding powdered calcium hydroxide (Ca(OH)2) to fresh water, usually RO. After mixing, the clear part of the solution is added very slowly to the aquarium to replace evaporational losses. When done correctly, this can raise the calcium and alkalinity level. When done incorrectly, it can cause violent pH increases and a drop in alkalinity. Special gadgets are available to aid both mixing and addition, which make it much easier to dose than it used to be.

The way kalkwasser reacts with other chemicals in the water is a little complicated, so the effect it has depends on the initial chemistry of your system. Kalkwasser reacts with CO2 present in the water and adds calcium and bicarbonate ions. If there's plenty of CO2 it produces calcium bicarbonate (which boosts the alkalinity). If there's not enough CO2, or if the pH is too high, some of the calcium precipitates to calcium carbonate, which is less soluble. Over time, this will lead to a drop in alkalinity, which you'll need to counter through the addition of buffers like sodium bicarbonate.

It can be an effective means of maintaining good calcium and alkalinity levels, but some reckon it's much harder to get these to levels beyond those seen in natural sea water, especially if the rate of evaporation is low and the calcium demand is high. There is also some evidence to suggest it causes significant internal differences in coral growth. It can be dodgy in the wrong hands and is really something for the more experienced reefkeeper.

Calcium chloride additives

Most of the additives on the market are probably based on anhydrous or dihydrate forms of calcium chloride - either as liquids or powders. On its own, calcium chloride pushes up calcium levels, but it can eventually cause a drop in alkalinity that needs to be countered through the addition of buffers. Some products contain little else apart from calcium chloride, while others may be combined with buffers, like sodium chloride, to help boost alkalinity.

Calcium chloride-based additives can be useful in tanks with medium or low calcium demand and are often used in conjunction with other additives, like two-part additives and kalkwasser. Calcium chloride-based additives can alter the chemistry of the water over time, particularly if few partial water changes are made.

Our test results suggest that several of these also contain additional magnesium (no bad thing in itself) - although this isn't always mentioned on the packaging.

Two part additives

Balanced two-parters, like C-Balance and Kent's Tech CB, or three-parters like ab Aqua Medic's Reef Calcium, are a clever and effective way of maintaining good levels of calcium and alkalinity. They add calcium and alkalinity in balanced ratios without messing up the balance of the water chemistry. So we think these are the safest bet, if a calcium reactor is out of the question.

The individual parts are kept in separate bottles to prevent them from reacting and forming insoluble chemicals. Essentially, when mixed together they form a sort of salt mix with a very high calcium level. They always come in liquid form - if they came as powders moisture in the air would allow the formation of insoluble calcium carbonate.

Some reefkeepers use calcium chloride-based additives to provide an initial boost, or to make minor adjustments, and then maintain those high levels using two-part solutions. Some of these may cause the s.g. of the water to rise a bit, so keep an eye on it. The cost of use may be higher than some calcium chloride supplements, but remember that you're actually dosing more than just calcium, and everything should be properly balanced.

What to aim for

Opinions differ on optimum levels, but they should always meet, or slightly exceed, those seen in natural seawater:

  • Calcium 400 - 420mg/l
  • Alkalinity 7 -10 dKH/ 2.5 - 3.5 meq/l
  • Magnesium 1200 -1300mg/l
  • pH 8.2-8.3
  • Salinity 35-36ppt


  • Don't dose without testing the calcium, magnesium, alkalinity and pH first. Even if the manufacturer of the supplement you're using suggests adding the treatment daily or weekly, you may not need to. The amount you need to add varies according to the calcium demand of your system.
  • It's becoming more common to raise calcium and alkalinity levels higher than those of natural seawater. But if you raise them too high, calcium and carbonate will precipitate.
  • Magnesium levels are particularly critical when you're dosing with calcium. They should ideally be between 1300 -1500ppm - any lower and it may be more difficult to raise the levels of calcium.
  • Slightly higher salinity levels allow you to maintain higher alkalinity and calcium levels.

Our tests

We undertook a controlled experiment on a range of 18 different calcium additives to examine their short-term effects upon water chemistry, and performed a number of calculations to determine their concentration and cost of use for a typical reef tank.

These tests showed only the short-term effects. Chemical and biological interactions in an aquarium during long-term use may have different results.

We used aquarium test kits, which while less accurate than laboratory grade equipment, give a basic representation of what's happening, although they will have undoubtedly missed some subtle changes. They also reflect the realistic conditions under which these products are likely to be used. Few reefkeepers test the chemistry of their water in a lab. Tests using lab grade equipment and different salt water would clearly give different results...

We calculated cost of use data based on the recommended doses required for a hypothetical average reef tank of 120 x 38 x 46cm (48" x 15" x 18"). After 25% of the water has been displaced by sand and live rock this holds around 160 l./35 gal.

Unfortunately, few companies were willing to divulge information regarding the composition and concentration of their products as many deem it commercially sensitive data. So we have not been able to look at the claimed concentration in every case.

Our results

Based on the results we obtained in the test, which are specific to our test water, we calculated some statistics on cost of use by examining the cost of supplying a daily calcium demand of 10ppm on our hypothetical 160 l./35 gal. tank.

Due to the widely differing concentrations of these products, and the differences between the claims made on the bottle and the actual concentrations we achieved in our tests (which could be due to the initial chemistry of our test water), we found some startling differences in the cost of use of these products. Based on the water we used for our tests, the actual cost of most treatments would be between £50 - £110 per year; the cheapest cost just £20 a year, while some of the less concentrated ranged from £150 - 435, right up to more than £1000!

But the prices quoted in this review for the individual products are based on the manufacturers' recommended doses, not the actual calcium demand of our 160 l. tank. Nor do they take into account the concentration of the products on test, which varies considerably.

Clearly, because there's a link between the levels you get and the chemistry of the water you're adding the supplements to, any results you get on your water will differ from ours. As a result, we'd recommend buying a small container of your chosen supplement and testing it on your own system to see how well it performs. You'll need to base your cost calculations around this.

Seachem Reef Complete

Reef Complete is a blend of ionic calcium designed to boost calcium levels to those found in natural seawater without having negative effects on pH, and is claimed to provide strontium and magnesium at levels equivalent to the rates at which they're depleted.

Seachem provide the breakdown of the product on the bottle, which is nice to see. The make-up consists of 16% calcium, 0.8% magnesium and 0.016% strontium. We calculated this to work out at 160,000 mg/l, or some 560 times more calcium than that available from an equal volume of kalkwasser, plus 8000 mg/l magnesium and 160 mg/l of strontium.

It comes with good, basic dosing instructions and gives three different treatment regimes based on the reefkeeper's level of experience (beginner, advanced or expert). Very wisely, Seachem suggest regular testing and adjusting the dose according to the calcium level of the tank.

The basic maintenance dose for beginners is 5ml per 80 l./17 gal., added twice weekly. This should raise the calcium level by around 10 mg/l each time. Based on this dose rate,1ml will treat 16 l./3.5 gal., so a litre will treat 16,000 l./3524 gal.

The expert instructions come with a basic formula to allow you to determine how much you need to add to raise the calcium by a set amount, which is very useful. Reef Complete comes in four sizes: 100ml (£5.20), 250ml (£8.40), 500ml £13.95 and 2 l. (£41.40). It pays to go for the largest bottle size if you are a heavy user since the price per ml varies from 5.2p (200ml bottle) to 2.07p (2 l. bottle). Based on the recommended dose, for a 160 l./35 gal. tank, regular bi-weekly doses would cost between £54.08 and £21.53 depending on the bottle size used.

Seachem Reef Advantage Calcium

This is a blend of powdered calcium and carbonates and claims to raise and maintain levels to values similar to those in natural sea water. It's claimed not to deplete magnesium or alkalinity, and boosts strontium and magnesium at typical utilisation ratios. Again, this product comes with a breakdown of constituents. It's claimed to contain 347,000mg/l of calcium (34.7%), 173,000mg/l of magnesium (1.73%) and 346mg/l of strontium (0.0346%).

There are several different dose rates: the basic dose is 5ml (one teaspoon) per 150 l./33 gal. twice weekly. Based on the recommended dose, we estimated running costs for the 250g container at a standard dose at about £21 each year for our 160 l./35 gal. tank. It comes in four sizes: 250g (£9.70), 500g (£15.90), 1kg (£27.30) and 4kg (£91.70).

Seachem Reef Calcium

Reef Calcium is a polygluconate complexed liquid calcium supplement designed to increase and maintain the calcium level without affecting the pH. It's claimed to be "bio available"; this means it's presented in a form easily utilised by corals and other invertebrates and additionally provides them with metabolic energy.

Polygluconate is a derivative of glucose and is supposed to keep calcium in solution in higher concentrations than without it. However, some experts claim that gluconates encourage algal blooms.

Reef Calcium contains 5% calcium (50,000 mg/l) and each dose should raise the calcium level by 3 mg/l. A normal dose for maintaining the calcium level is suggested as 5ml per 80 l./17 gal. As a rough guide, this should maintain a stable level if the calcium drops by about 24 mg/l per month - which is probably adequate in the average reef tank but far less than the amount required in an SPS system with a much higher calcium demand. Based on the recommended dose a 160 l./35 gal. tank would cost a maximum of £54.08 per year.

The dose can be increased to as much as 15ml per 80 l./17 gal. per day if the demand for calcium is very high - this should provide enough for a tank with a calcium demand of about 9mg/l per day. However, if this is the case, your money would be better spent on a calcium reactor since the cost of use of Reef Calcium under these circumstances would be prohibitive at more than £200, even when using the largest and cheapest bottle available.

Reef Calcium comes in four sizes: 100ml (£5.20), 250ml (£8.40), 500ml (£13.95) and 2 l. (£41.40).

Tropic Marin Bio-Calcium

This powdered supplement contains calcium, hydrogencarbonate and trace elements in balanced proportions. The instructions recommend keeping the calcium level below 420ppm to prevent the precipitation of calcium carbonate, so it's not the ideal supplement to use if you want really high calcium levels.

The maximum recommended dose rate of 1 level measuring spoon per 18 l./4 gal. is designed to raise the calcium level by approximately 28ppm. Unfortunately, the instructions don't state how much the whole container will treat, which makes it impossible to work out exactly what you're paying for in the shop. TMC told us that one measuring spoon contains approximately 9.2g of Bio-Calcium, which means that the 500g pot will treat just under 1000 l./220 gal. and raise the calcium level by 28ppm.

Our hypothetical 160 l./35 gal. tank would need about 82g to raise the calcium level by 28ppm, which would set us back around £1.69 per dose, which is quite expensive. The frequency of dosing would depend on the rate of calcium depletion in your tank. As a guideline, weekly use on a 160 l./35 gal. tank would cost around £87 a year. Tropic Marin Bio-Calcium comes in two sizes: 500g (£10.33) and 1.8kg (£23.97).

Salifert Coral Calcium

Coral Calcium contains 160,000 mg/l of calcium, which is claimed to be the maximum amount it's possible to get in a liquid supplement, and equates to a concentration of about 16%. It also contains sodium, chloride, magnesium, sulphate and bromide of pharmaceutical grade (or higher) in balanced quantities.

It's designed for boosting the calcium levels, as well as being a long term supplement, and needs to be added at a rate of 5ml per 100 l./22 gal. This causes an increase of 8ppm in calcium level. Very wisely, the instructions suggest only raising the calcium level in small steps of less than 50ppm per day.

Increasing the calcium level by 8ppm on a 160 l./35 gal. tank via a one-off dose would cost about 27p, while raising the calcium level by 40ppm would cost about £1.34. Based on the recommended dose, regular weekly doses would cost just under £14 a year, and would cater for a weekly drop in calcium of around 8ppm - not expensive, but only a very small calcium demand. Coral Calcium comes in three sizes: 250ml (£8.37), 500ml (£14.08) and 1 l. (£23.79).

Salifert All in One

All in One, as the name suggests, increases the calcium, alkalinity and trace element levels simultaneously. It contains 35,000 ppm, or 3.5%, calcium, plus strontium (700ppm or 0.07%) and other trace elements including zinc, manganese, cobalt and iron.

The calcium is present as calcium acetate, which Salifert claim is quickly converted to calcium bicarbonate. It also contains additional amino acids used by corals during growth.

It's designed to be dosed weekly at 5ml per 100 l./22 gal. Based on the recommended dose, a 160 l./35 gal. tank would cost around £14 per year, but it provides less calcium than Coral Calcium. Salifert state that additional treatments with alkalinity buffers like its KH + pH Buffer may be necessary during treatment. All in One comes in three sizes: 250ml (£8.37), 500ml (£14.08) and 1 l. (£23.79).

Salifert Coral Grower

Salifert Coral Grower contains a blend of calcium and strontium along with a balanced mix of sulphate, chloride, bromide, sodium, magnesium and trace elements. It's fairly concentrated and contains around 80,000 mg/l calcium (8%) and 1600 mg/l (0.16%) strontium and claims to be produced from pharmaceutical grade raw materials or higher.

Salifert recommend dosing weekly at a rate of 5ml per 100 l./22 gal., which means that 1 l. treats 20,000 l./4405 gal. Based on this dose, the smallest bottle size would cost around £13.93 to treat a 160 l./35 gal. tank each year. Salifert recommend using it in conjunction with their KH + pH Buffer to supply carbonates. It comes in three sizes: 250ml (£8.37), 500ml (£14.08) and 1 l. (£23.79).

Rowa Life Elements Calcium +

Life Elements Calcium + is a new product marketed under the Rowa brand by D and D Aquarium Solutions. It's a typical calcium chloride additive, which also contains some additional magnesium to improve performance. At the time of going to press, D and D Aquarium Solutions hadn't decided on a retail price for this product, though we were given a guide price of £19.90 per litre. The recommended dose rate of 10ml per 100 l./22 gal. twice weekly would cost 32p per dose, or about £33 per year.

Red Sea Reef Success Calcium+3

Reef Success Calcium+3 contains additional strontium, molybdenum and iodine and needs to be mixed with freshwater and used to compensate for evaporational losses, like kalkwasser. It shouldn't be poured into the main tank in its undiluted form. It doesn't come with any data on the constituents or concentration, so you'll need to see what happens in your own system using the recommended doses. It needs to be dosed at a rate of 5ml per 120 l./26 gal. of aquarium water, once each week. On a 160 l./35 gal. tank this works out at £11.69 per year. Reef Success Calcium+3 costs £13.99 for a 415ml bottle.

Red Sea Reef Success Calcium

Reef Success Calcium is a liquid solution which claims to raise the calcium without affecting the pH or alkalinity. It's also said to be a balanced supplement and won't affect the sodium and chloride balance of the water, even after prolonged use. Again, no exact concentration details are given, but Red Sea do give two different dosages, one for increasing the calcium level in jumps of 50ppm, and one to maintain a stable calcium level.

To increase the calcium level by 50ppm you need to add 15ml per 20 l./4.4 gal over a period of four days. The dose rate here works out at 1ml per 1.3 l./0.3 gal., making the cost around £12.71 for the four-day course on a 160 l. system. At the weekly maintenance dose of 5ml per 120 l./26 gal. the dose rate is about 1ml per 24 l./5.3 gal. This works out at about £9.18 a year which looks very cheap, but this dose only provides a fairly small amount of calcium. A 415ml bottle of Reef Success Calcium costs £10.99.

Red Sea Reef Success Calk

Reef Success Calk is claimed to be a balanced supplement aimed to replenish calcium and carbonates in the right proportions at the same rate as they're removed by invertebrates. It's a maintenance supplement designed for regular daily or weekly use on water with an initial calcium level of 380 - 420ppm. The recommend technique for dosing Calk is to monitor the calcium and alkalinity levels daily for a week to monitor their decline and determine the approximate rate at which they are being utilised by the inverts and algae present.

It's dosed at a rate of around 0.5g per 100 l./22 gal., but you need to test the alkalinity regularly and make adjustments to the amount added depending on the results. No details on constituents or concentration are provided so we can't comment on how much it's likely to cost to use or how effective it is at maintaining balanced calcium and alkalinity levels.

It sells for £11.99 for 500g, and comes with a free Alkalinity Pro test kit. Calk should be used in conjunction with Coral Trace.

Kent Turbo Calcium

Turbo Calcium is one of Kent's Expert Series product range and is based on 33-37% anhydrous calcium chloride and is designed for maintaining a high calcium level. It needs handling carefully as it can get hot when it's mixed with water. Turbo Calcium comes in three sizes: 200g (£12.25), 400g (£18.75) and 800g (£25.25). This works out at between 3.1 and 6.1p per gram, depending on the container size used.

Kent Liquid Reactor

Kent Liquid Reactor is a new supplement that the manufacturer claims will "give the same results as a costly calcium reactor" by boosting calcium, alkalinity, magnesium and strontium levels. Like Seachem, Kent provide a breakdown of the ingredients.

Liquid Reactor claims to provide 126,000 mg/l (12.6%) calcium and 193,000 mg/l (19.3%) carbonates, so it's a bit more concentrated than most other liquid supplements on the market.

It needs very thorough shaking before use as some of the contents settle. However, one shop we spoke to mentioned that they'd had problems resuspending the material through shaking alone.

Depending on the dose used, a single treatment on a 160 l./35 gal. tank costs between 21-42p or 13-25p when the largest bottle size is used. Annual running costs depend on the calcification rate of the tank, and the dose rate used, but vary between £26.38 and £43.96 when used twice weekly. It clouds the water for an hour or so after addition and Kent strongly recommend that it's added at night, so you don't have to look at a cloudy tank and so that it can prevent nightly pH drops. Liquid Reactor comes in two sizes: 473ml (£24.99) and 1892ml (59.99).

Kent Tech CB Part A and Part B

This liquid two-part solution, as the description suggests, comes in two separate bottles.

Two 1 l. bottles treat 15,100 l./3326 gal. You need to add a dose every day until the target calcium and alkalinity levels are reached. It's fairly concentrated, 10ml of each part is needed per 151 l./33 gal. Based on the recommended dose, a one off dose using both bottles on a 160 l./35.2 gal. costs about 40p. If you need to treat the tank every couple of days it would cost around £42 a year. Daily use would set you back about £147 a year. Kent Tech CB comes in two sizes: 16oz Part A and B (£17.98) and 64oz Part A and B (£43.98).

Kent Liquid Calcium

Liquid Calcium contains a minimum of 10% calcium as calcium chloride and is described as "bio available". The dosage instructions could be a touch confusing for UK reefkeepers, since they're given in teaspoons and US gals, as well as milliliters, with no conversion information. In a nutshell, a daily dose of 1.25 - 5ml treats 189 l./41 gal. (50 US gal.), depending on the calcium demand of your tank.

For our hypothetical 160 l./35 gal. tank, daily doses using the 16oz (473ml) bottle would cost around £50 each year. At the maximum dose it would treat around 37,800 l./8326 gal.

Kent recommend using it in conjunction with Marine Superbuffer dKH or Pro-Buffer dKH and Strontium and Molybdenum. It comes in three sizes: 8oz (£8.59); 16oz (£15.50) and 64oz (£40.75).

ab Aqua Medic Reef Life Calcium

Reef Life Calcium is a balanced three-part solution. It comes in a boxed set containing three separate treatments (calcium, trace and buffer tabs), designed to be used in conjunction with each other to raise the calcium, alkalinity and trace element levels. It raises the calcium level using calcium chloride, and raises alkalinity using sodium bicarbonate. Ordinarily, this might interfere with the chemistry of the water by raising the salinity through an increase in sodium chloride. However, the extra bottle adds trace elements in exactly the same proportions as Aqua Medic's Reef Salt. This means that although there's still a slight increase in the salinity, it's not simply due to the presence of additional sodium, which is much better for the chemistry of the system, and easily rectified.

What you actually get for your money isn't stated on the packaging. According to ab Aqua Medic, the separate bottles provide 162g Ca(HCO3) and 168g of ab Aqua Medic Reef Salt.

Dr Manfred Schluter of ab Aqua Medic told us: "The corals and other calcium consumers take up the Ca(HCO3) and transform this into calcium carbonate to build up their skeletons, while the CO2 is used by the zooxanthellae for photosynthesis. The Aqua Medic Reef Salt component will produce an increase in the salinity. But, usually, this is not more than the loss of salt water from protein skimmer waste."

For a basic dose, you need to add 15 tablets and 15ml of each liquid per 400 l./88 gal. each week. This works out at about 6ml of each liquid and six tablets for a 160 l./35 gal. tank, or just under £20 per year. Instructions are also provided to allow you to calculate the amounts required to raise the alkalinity to a desired level. Reef Life Calcium includes: Reef Life Buffer Tabs (250 tablets), Reef Life Calcium (250ml) and Reef Life Trace (250ml) and sells for £16. This looks like one of the better products on the market.


Kalkwasser products are simply powdered calcium hydroxide. Despite what the manufacturers may tell you, there's really not a huge amount of difference between them, although some may contain more impurities than others. So, we've not tested them in this review. Those with fewer impurities tend to command a higher price. Theoretically, these may be a better purchase, because it's difficult to tell exactly what "impurities" you're adding. However, it's also said that kalkwasser can precipitate many impurities out of solution (including phosphate) - although some chemists dispute that this occurs in saltwater. These impurities are said to settle out in the white scum that forms when the kalkwasser is mixed, but aren't present in the clear component of properly mixed kalkwasser.

Rowa Kalk

This is a new calcium hydroxide additive from D and D Aquarium Solutions. They claim that this is purer than competing brands on the market at around 99.1% calcium hydroxide.

It comes in two sizes: 500ml (ca. 325g) (£12.95) and 1000ml (ca. 650g) (£17.99). A bulk 5 l. (ca. 3kg) container is also available, which sells for around £42. Prices range from 1.4 to 3.98p per gram, depending on the container size used - the cheapest on review here.

Seachem Reef Kalkwasser

This calcium hydroxide product also claims to have unsurpassed purity and solubility. It comes with easily understood instructions and dosing information. It comes in three sizes: 250g (£10.50); 500g (£13.20) and 1kg (£25.50). This works out at between 2.55 and 4.2p per gram.

Kent Marine Kalkwasser Mix

Again, the distributor claims this product is purer than competing brands. It comes with good basic instructions on how to dose safely and what to test for. Kalkwasser Mix comes in three sizes: 100g (£9.25); 225g (£13.75) and 450g (£18.75). This works out at between 4.16 and 9.25p per gram - twice as expensive as some competing brands in smaller pot sizes.

Two Little Fishies Kalkwasser

Two Little Fishies' Kalkwasser comes with good instructions which explain most of the things to look out for when dosing, and also provide some handy tips on using it in conjunction with carbon dioxide. It comes in two sizes: 454g (£17.99) and 1.8kg (£49.95). This works out at between 2.8 and 4p per gram.

Aquarium Systems Reef Evolution Calcium Hydroxide

Reef Evolution Calcium Hydroxide (distributed by Underworld) comes with clear instructions that emphasise the importance of testing, and of using the clear part of the mixed kalkwasser. It's available in two sizes: 227g £10.45 and 454g (£17.75). These work out at between 3.9 and 4.6p per gram.

Two Little Fishes C-Balance

C-Balance, from Julian Sprung's Two Little Fishies company and sold in the UK by Underworld, is a two-part solution. It comes with a breakdown of the contents, which is good to see, and it claims to adjust the calcium, alkalinity, magnesium and strontium levels in balanced proportions. The standard dose rate is 5ml of each part per 75 l./16.5 gal. per day. The maximum recommended dose you can use is 1ml per 3.8 l./0.8 gal., which needs to be added in two separate additions each day. Daily doses at the standard rate would work out rather expensive on our hypothetical 160 l./35 gal. tank at about £164 each year - making a calcium reactor look tempting in this case. However, several of the reefkeepers we spoke to were very complementary about the composition and performance of this additive during long term use, and it's considered one of the better products on the market for safe, balanced dosing. C-Balance costs £19.99 for the two 474ml bottles.


Casco (Seachem) - 07000 393940

Underworld (Aquarium Systems) - 01509 610310

D and D Aquarium Solutions (Rowa) - 0208 5012492

TMC (Tropic Marin and Salifert) - 01923 284151

ab Aqua Medic - 0845 090 3500

Aquatic Solutions (Kent Marine) - 01553 776788

Interpet (Red Sea) - 01306 881033

Matt Clarke

Copyright (C) Matt Clarke/Practical Fishkeeping magazine, phone 01733 465204 - fax: 01733 465246 -

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