A Kalkreactor or Kalkwasser Reactor is a unit which supplies calcium to your aquarium in a liquid form. Definition: kalk·was·ser * Chemical Solution > Made from calcium hydroxide mixed with water, it is a strong calcium solution added to reef systems. Also referred to as limewater. The kalkreactor aids in promoting a beautiful tank with rapid growth of coralline algae. Calcium is an important and essential reef tank additive. Most kalkreactors are equipped with a circulating pump which mixes the kalkwasser before it is dosed into your tank.