Calcium (Ca)

Calcium is the fundamental building block for the growth of corals and invertebrates. In the reefs it is consumed continuously and must be constantly replenished.

Detailed explanation

Calcium dissolved in sea water is essential for all organisms that build calcareous structures: SPS and LPS hard corals, clams, coralline algae and even some tube worms. In nature, calcium comes from the leaching of rocks. In the aquarium we have to provide it ourselves.

Calcium consumption depends on the density of corals in the aquarium. An aquarium with few soft corals consumes very little calcium. A reef full of Acropora can drop calcium by 20-30 ppm per day.

Calcium works in balance with KH and magnesium. If you supplement calcium without balancing KH the system crashes. This is why the Balling method administers the three elements in proportion.

In freshwater aquariums, calcium plays a different role. It contributes to general hardness (GH) and is used by gastropods to build their shells. In koi ponds, calcium helps the formation of scales. For brackish aquariums, intermediate levels between freshwater and marine aquariums are needed.

Reference values by aquarium type

How it is measured

Salifert or Red Sea drop test for marine aquariums. ICP analysis for maximum precision. In freshwater aquariums it is usually measured indirectly via GH.

What happens when it is too low

The corals stop growing. Coralline algae fade. The clams close the mantle. Gastropods develop thin, fragile shells.

What happens when it is too high

Above 500 ppm in marine aquariums there is a risk of spontaneous precipitation with milky water. Excess calcium competes with magnesium and other ions.

Related terms