The equilibrium between Calcium and Carbonic acid in the seawater aquarium is often disturbed in seawater tanks, due to the biological activity of animals and plants. The organisms take off calcium carbonate for the construction of their skeletons. The result is a shortness of Calcium and an increase of the pH. The Aqua Medic Calcium Reactor is a module for the creation of a natural calcium carbonic acid equilibrium in the aquarium. It is filled with hydrocarbonate. As calcium carbonate is not soluble at pH values, that are recommended in seawater aquaria, CO2 has to be injected into the reactor. This results in a local decrease of the pH and the solution of Calcium hydrogen carbonate. This way of calcium dosing does not effect the pH, like the use of "Kalkwasser". Calcium hydrogen carbonate is soluble and is taken by plants and animals for their skeletons. They convert it back to calcium carbonate again.