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.
Kalkwasser contains no other ions that would otherwise cause an imbalance in the water chemistry.
If the evaporated water loss is from the aquarium is replaced with kalkwasser then it will add calcium and offer the other benefits described above. The solution however is highly reactive with CO2 in the air forming insoluble calcium carbonate. The reaction with CO2 significantly reduces the free calcium ions from 900mg/l in the fresh solution at pH 12.4 to 5-6mg/l at pH 10.
For this reason it is necessary to continually make a fresh solution and to keep it away from the atmosphere. This is where a Deltec Kalkwasser stirrer may come in to play as it makes up fresh saturated solution as it is used up.
Note that great care must be taken when administering Kalk due to the great affect that it has on the pH of the aquarium.
Kalkwasser is ideally dosed after the lights have gone out on the aquarium to counter the natural drop in pH which occurs when photosynthesis stops.