Corals, clams and certain types of algae have to have calcium in order to grow. They consume calcium and can quickly deplete the supply. Ocean water has a calcium level that ranges from 380 - 450 mg/L. It can be difficult or impossible to keep the levels this high in a home aquarium. But, there are several ways that you can increase the calcium levels. One of the most popular ways is by using reactors.
Calcium reactors can be used to add dosages of calcium to your aquarium. And, Kalkreactors can add dosages of Kalkwasser. If you prefer you can also use a diluted calcium chloride to dose, but it is more expensive. Liquid calcium also poses a problem. You will need to use carbonate or bicarbonate as a buffer because you are taking the risk of lowering the levels of alkalinity. If this method is something you are interested in, you may want to consider using a two part calcium/alkalinity solution.