Kalkwasser is a German word and is used by marine aquarists to denote a solution of calcium hydroxide (or, occasionally, calcium oxide). The terms “kalkwasser”, “lime water” and "calcareous water" are interchangeable. Kalkwasser is often abbreviated to "kalk".

Kalkwasser is an economical and effective way to maintain calcium and alkalinity levels in a marine aquarium. Calcium is used in reef tanks by calcifying organisms, and is depleted over time. In tanks with many such organisms, water changes are unlikely to maintain high enough calcium levels and supplementation is therefore required. Kalkwasser is one such method. In tanks with moderate to high calcium demands, kalkwasser may need to be supplemented with a calcium reactor.

Kalkwasser has been widely used in reef aquariums since the mid 1970s. There are three main advantages to kalkwasser over other methods of calcium supplementation:

First, it does not leave any toxic or harmful residue in the aquarium.
Second, it precipitates phosphate from the water.
Third, it is a balanced supplement and will maintain alkalinity levels.
It has also been suggested that the hydroxide ions released from kalkwasser will react with heavy metals, such as copper, and remove them from solution if they are present in the aquarium