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Water Parameter
Unit(s): 1 ppm ~ 1 mg/l
Target: 400 - 450 ppm
Danger: < 200 ppm
Tendancy: Decrease
Increased: Additive

Water changes

Decreased: Calcification


Testing: Weekly
Maintenence: Calcium chloride

Calcium hydroxide

Calcium hydroxide reactor

Calcium carbonate reactor


Calcium (along with alkalinity) is an important element in any marine aquarium where calcareous organisms are present. These are organisms that secrete calcium carbonate and include:

Normal seawater has a calcium concentration of around 420 mg/L and the ideal concentration in a marine aquarium should be around this number or slightly higher.

Maintaining Levels

The easiest way to maintain calcium (and alkalinity) in an aquarium is to replace evaporated water with kalkwasser. This involves dissolving calcium hydroxide in water which is then added slowly to the system via a drip, peristaltic pump, or float valve system. This can also be added using a calcium hydroxide reactor.

For aquaria with a higher demand for calcium and alkalinity, a calcium carbonate reactor can be used. This uses carbon dioxide to lower the pH of the water that enters the reactor, which then dissolves the calcium carbonate substrate. This effluent which is enriches in calcium and alkalinity in the correct ratio is then returned to the system.

It is also possible to adjust calcium levels directly by using calcium chloride or similar products. It is available in various specific commercial aquarium preparations such as Kent Marine Turbo-Calcium. While not strictly calcium chloride, Seachem Reef Calcium servers the same purpose. Damp Rid which can be purchased in most supermarkets is also calcium chloride. Some commercially available produces have the additive already dissolved in water. While this may be convenient, it is a very expensive way to maintain levels.

Long term use of the calcium chloride products should be avoided as they not only add calcium but chloride (or some other anion) which will accumulate without regular water changes. Kalkwasser and calcium carbonate reactors add nothing else but calcium and alkalinity so are called balanced additive techniques.