Kalkwasser in the marine aquarium

The main benefit of Kalkwasser use in the aquarium is that it replenishes calcium in the reef and marine tank.

Even the most inexperienced hobbyist can easily make Kalkwasser. Kalkwasser is extremely beneficial for corals, coralline algae, and many reef inhabitants. It also helps raise the pH level, which is needed in a marine and reef environment.

It might come as a surprise that Kalkwasser, which is German for lime, does not contain any lime at all. The name comes from the main compound used: Calcium hydroxide.

Calcium hydroxide is available in powdered form and contains calcium, oxygen and hydrogen. Calcium hydroxide is mixed with R/O water, where parts of the chemical combination is broken down. Calcium hydroxide is not easily soluble in water, but will break down to a degree. The components that break down are calcium and hydrogen ions. Calcium hydroxide will fall out and collect at the bottom. The broken away parts collect themselves in the clear water above. The residue can be reused with fresh R/O water to create a new batch of Kalkwasser.

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In short Kalkwasser is calcium and hydrogen ions with water as a result of mixing calcium hydroxide with R/O water. Kalkwasser has a pH of about 12 – 12.5.

The next step takes place in the aquarium itself. Present carbon dioxide (CO2) will react with the Kalkwasser (calcium and hydrogen ions) to create calcium-hydrogen-carbonate. Calcium-hydrogen-carbonate is highly unstable and breaks down into calcium and carbonate. The benefits of calcium are outlined above and the carbonate helps keep the pH stabile (carbonate hardness).

Because of the reaction with carbon dioxide Kalkwasser is hard to dose, since it is dependent on the CO2 content in the aquarium. This is a good reason to get rid of airstones as they will dilute the CO2 rendering the CO2 and Kalkwasser useless by diffusion.

The best way to introduce Kalkwasser is as top off water for the aquarium. As pure water evaporates over time, adding the Kalkwasser solution will replenish the much needed calcium and pH buffer.

Knowing about the high pH of Kalkwasser, it is essential to monitor the pH level while adding it. Kalkwasser is best introduced in the evening since pH levels vary throughout the day, measuring their lowest in the morning. Adding Kalkwasser in the evening will therefore have less influence in the beginning and helps the system to adjust better and to avoid possible pH “shocks” (as in a to sudden pH increase).

Kalkwasser is recommended, if not essential for reef tanks

Alternatively a kalkwasser reactor can be used. Below is a brief overview:

The main medium for kalkwasser reactors is lime (CaCO3). Crushed sea shells are commonly used. Avoid crushed corals as they will release phosphates during the process. Since lime will not dissolve in high pH levels, the pH has to be lower than 7. This is achieved by adding CO2 into the reactor, which will lower the pH to more favorable conditions. As lime can now be dissolved, calcium-hydrogen-carbonate is created. This has the same effect as adding kalkwasser to the tank as described above.

If a reactor is used, it is counter productive to add Kalkwasser. In fact they don’t work well together. The output of calcium and hydrogen ions can be controlled through the reactor. Adding Kalkwasser will create calcium at a rate that can turn the water cloudy (calcification). Only one of the methods should be used.