In a closed system aquarium with a heavy load of hard corals, it is hard to maintain a necessary level of calcium for proper exoskelton growth of hard corals. Water changes only replenish a small amount of calcium and then the remaining is either added via a calcium reactor or additives.

There are different forms of calcium additives availble on the market, and the individual aquarium determines which is best suited. For instance if you have trouble maintaining a normal ph you may want to consider a kalkwasser drip, or if your parameters are correct and you need to add calcium without affecting the system then calcium chloride will be more suitable.

Upon mixing calcium chloride, a little bit of heat is created due to the chemical mixing with water. For making your own two part solution a good reference is Randy's two part recipe. He gives a step by step method of producing your own two part recipe.