Reef waters contain between 400 and 500 mg/liter of calcium, which is up to 20 times more than fresh water. Calcium participates in the composition of calcium carbonate (CaC03), necessary for the formation of coral skeletons, the carapace of crustaceans, and the shells of mollusks. If you are raising these animals, the CH must be between 6.7 and 14°GH; if it is below this level, part of the water must be changed or calcium must be added. This is not necessary if there are already fish in the tank.
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