Two Little Fishies Kalkwasser by Julian Sprung is a high-purity calcium hydroxide (Ca(OH)₂) powder used to maintain calcium and alkalinity in marine reef aquariums. When mixed with purified freshwater, it creates kalkwasser (“limewater”), which is typically dosed through an auto top-off (ATO) system or a kalk reactor to replace evaporated water. Key benefits Maintains calcium and alkalinity levels in growing reef tanks. Helps stabilize pH, especially when dosed slowly at night. Supplies calcium and alkalinity in the same ratio that corals and coralline algae consume them. Can help reduce phosphate by promoting precipitation of phosphate compounds. Best practices Mix only with RO/DI water. Dose slowly to avoid sudden pH spikes. Allow the solution to settle and dose only the clear liquid, leaving any undissolved powder at the bottom. Store the mixed solution in a sealed container to minimize exposure to air, which reduces its potency. Kalkwasser works well for many reef aquariums with low to moderate calcium and alkalinity demand. Tanks with heavy SPS coral growth may still require additional supplementation, such as a two-part dosing system or a calcium reactor, to keep up with consumption.