Measuring calcium in reef tanks - what for?

All animals that form a calcium skeleton or a calcium shell require both calcium and carbonate ions for this purpose. A sufficient supply of these two ions therefore plays an important role, especially for the growth of stony corals in your tank.

In the oceans the average calcium concentration is about 420 mg/l. The calcium concentration in your aquarium should ideally be in the range of 400 to 450 mg/l. Note that the formation of lime, i.e. calcium carbonate, can only take place if the concentration of carbonate ions is high enough. This is the case when the alkalinity of your tank is in the range of 7.0 to 9.0 °dH.

Due to the calcification processes taking place in your aquarium, the calcium content and the alkalinity are continuously decreasing. By regularly measuring both values, you should monitor this process and, if necessary, intervene to regulate it. To maintain the calcium and carbonate concentration, different supply systems have proven themselves in practice (e.g. Balling method, Carbo-Calcium, All-for-Reef & Calcium Reactor).

Adjusting calcium
As mentioned before, calcium ions are consumed during the calcification process, so you need to add calcium to your aquarium at regular intervals.

To increase the calcium concentration you can either use ready-made products or prepare your own stock solution.

For this you have to dissolve 800 g calcium chloride dihydrate (CaCl₂*2H₂O) in one liter of osmosis water. 4.59 ml of this solution will increase the calcium concentration in 100 liters of your aquarium water by 10 mg/l.

For example, if you want to increase the calcium concentration in your 300 liter aquarium from 400 to 420 mg/l, you must add a total of 27.54 ml of the stock solution. Increasing the calcium concentration should always be done slowly, which is why you should not add the required amount of solution to your tank all at once, but gradually.

Another method, in which you only increase the concentration of calcium ions in your tank, is the addition of Kalkwasser. However, here you cannot calculate or determine the amount of calcium ions added. The Kalkwasser method is associated with further disadvantages, which is why you should inform yourself about the various aspects of this method before using it.

When adding calcium ions you should keep an eye on the alkalinity, because calcium and carbonate ions are usually consumed in a fixed ratio. By using the so-called Balling method, Carbo-Calcium or operating a calcium reactor, these two ions are added to your aquarium in a balanced ratio.

Regular water changes also help to stabilize the calcium concentration.

If the calcium concentration in your tank is too high, you should temporarily stop adding calcium ions. Just wait until the calcium concentration has dropped to the desired value. You should always keep an eye on the alkalinity and intervene if necessary.