Kalkwasser is a solution made by dissolving Calcium Hydroxide in water. The resultant fresh solution is alkaline with a pH of 12.4 and contains a ready supply of both Calcium ions and Carbonate ions.

Both of these ions are useful to the aquarist as the Calcium ions are available to the corals and the Carbonate ions help to buffer the pH. In addition to adding calcium, Kalkwasser also has the benefit of adding hydroxide ions, which react to neutralise organic acids in the aquarium that otherwise would exhaust the buffering capacity of the system, thus countering the natural tendency within the aquarium towards falling pH. It also helps precipitate phosphates from the water, which fuel nuisance algae growth and inhibits calcification of stony corals.