In any aquarium, there is natural water evaporation. If you are using a high-quality protein skimmer and high flow rate pump, you will get even more evaporation than you would otherwise. As saltwater evaporates, we lose water but keep the salt. This slowly raises the salinity levels in the aquarium as water evaporates out.
It is important for the aquarist to add fresh water to replace what has been lost and dilute the rising salinity levels. When replacing evaporated system water it is also important to replace lost calcium levels. Calcium in the aquarium is absorbed by nearly all living things and is crucial to the health of corals and other invertebrates.
Evaporation, protein skimming, and the addition of make-up water all deplete calcium — that is where kalkwasser comes in. Kalkwasser is a German word that means, “lime water” or “chalk water.” It is also a trade name for calcium hydroxide.
Kalkwasser is typically mixed with make-up water to replenish lost calcium levels. It’s very important that aquarists keep reef aquariums not use plain fresh water as make-up water. Plain fresh water has a different pH level than saltwater aquarium water and also does not contain the needed calcium to replace what has been lost.
How much should you use?
Aquarists have to be careful when using kalkwasser. Overdosing with kalkwasser can cause severe problems because it contains a very high pH. Changing the pH rapidly in the aquarium can shock inhabitants, causing stress or in extreme cases death.
It is best to use a kalkwasser mixture within one day of making it as the product loses its effectiveness as time goes on. In addition to being used for make-up water, some aquarists slowly drip kalkwasser into the aquarium as a means of constantly replacing lost calcium. There are specialized reactors that are suited for doing this and it is best to have experience in dosing reef aquariums before attempting this method.
When should you use it?
One recommendation when adding kalkwasser to the aquarium is to do so early in the morning before the lights turn on. At night the pH in our aquariums typically drops a few points. If kalkwasser is added at this time, the rise in pH will not be as significant as it may be if the solution is added midday when the pH is at its peak. It is important not to use an air stone with make-up water containing kalkwasser. This adds carbon dioxide and therefore lessens the effectiveness of the calcium hydroxide.
Interestingly the addition of kalkwasser has a few fringe benefits above just replacing calcium and raising pH. Kalkwasser encourages the growth of pink coralline algae, buffers to maintain a stable elevated pH, and provides several valuable chemical reactions to the reef aquarium. Many of the pH/calcium buffers on the market utilize kalkwasser or have a technology that is based on it. So the short answer to your question is that kalkwasser is an alkaline product used to buffer pH and raise calcium levels in the reef aquarium.