Limewater goes by several names, including “kalkwasser” (German for “chalk” water or calcareous water). It is prepared by mixing calcium hydroxide in water. Most people use a saturated solution, which can be prepared easily and reproducibly by simply adding an excess of calcium hydroxide to water, and mixing it thoroughly. The solution is often allowed to settle, and then it is dispensed slowly, drop-wise, into a turbulent area of the system, preferably in a sump if one is present. In doing so, one gradually replenishes the calcium and alkalinity depleted by calcification in the aquarium. A kalkwasser reactor is a chamber to contain and mix calcium hydroxide to top off water. Within the chamber a pump mixes the settled calcium hydroxide into the water column. Top off water is pumped into the reactor chamber and consequently, calcareous water (kalkwasser) is forced out of the chamber and into the aquarium. Kalkwasser is delivered slowly and consistently. This delivery method can maintain calcium and alkalinity levels without the risk of a large pH spike.