Calcium is a chemical element, with the element symbol Ca
Calcium is extremely important for most reef inhabiting species, foremost however for stone algae and coralline algae. Reef forming stone corals (SPS) need calcium in great amounts for skeleton formation. LPS stone corals, however, have a slightly reduced need for calcium.
In salt water aquariums, calcium is especially relevant in the form of the compound calcium carbonate. Calcium carbonate consists approx. of 40% calcium and of 60% carbonate. Calcium / calcium carbonate has to be added repeatedly to the aquarium since it is metabolised by algae.
The regular measurement of the Ca content is extremely important for a reef aquarium. Please note, that at the same time the KH value (alkalinity) should also be measured. A consumption of calcium, as a result of the calcination to calcium carbonate, inevitably also leads to a reduced alkalinity.
The magnesium content of the aquarium water also influences the measurement result. Therefore, this value should always be measured at the same time along with the Ca an KH measurement and if necessary, be adjusted.
By the way: a good proliferation of coralline algae or white growth tips on stone corals are a good indicator for a sufficient calcium content.