Calcium (Ca2 ) is vital to many biological functions. In saltwater aquariums, calcium should remain in the proper ionic balance along with alkalinity (KH) and magnesium (Mg2 ). A calcium level outside of this ionic balance can cause poor coral skeleton growth and tissue necrosis. Calcium uptake in reef aquariums can change over time and should be regularly tested to ensure proper dosing.

Though more closely maintained in saltwater systems, freshwater aquariums also require routine monitoring of calcium. Plants require calcium for proper leaf growth and other functions. Invertebrates, such as shrimp and snails, require the right amount of calcium to maintain shell integrity and prevent molting problems.

Tropical community fish: 75 ppm calcium
African cichlids: 120 ppm calcium
Marine aquariums: 300 to 400 ppm calcium
Reef aquariums: 400 to 450 ppm calcium