Hard corals need calcium to build their skeletal structure and to grow.
The best way to add calcium into a closed environment such as a reef aquarium is with either calcium supplements such as Salifert Coral Calcium or kalkwasser, or alternatively through the use of a calcium reactor.
Generally a calcium reactor is filled with a media such as coral gravel, which has CO2 fed into the chamber gradually breaking the coral gravel down which allows the calcium from the gravel to be fed into the aquarium and assimilated by the corals.