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Calcium is a minor component of sea water, but is extremely important for the health of stony corals. It is this dissolved calcium that is extracted by the corals and used to make their skeletons. If this element is in short supply, tissue recession can occur, resulting in poor health and ultimately, death.

Target Value:
400ppm (380 - 480 ppm)
Note: values over 480 are not recommended as they can cause spontaneous precipitation of calcium carbonate (see Alkalinity - Interactions: calcium and alkalinity)

More Detail:
Stony corals constantly deposit new skeleton by combining calcium ions (Ca++) with bicarbonate (HCO3-) to form calcium carbonate.

Because of the constant removal of calcium from the water, levels must be constantly maintained by the aquarist... this is one of the more dubious pleasures of owning a reef tank.

There are three popular methods for calcium maintenance:
  • addition of kalkwasser
  • the use of a calcium reactor
  • the addition of two part supplements
Each method has its own pros and cons.

Interactions: Calcium and alkalinity (see alkalinity section)