Calcium is required for the formation of aragonite, the skeletal material of reef-building invertebrates such as corals, clams, tube worms, and their respective allies. In reef aquaria, maintaining calcium within a range of 412-450 ppm will enable corals and other reef-building invertebrates to grow rapidly when all other physical and chemical requirements are met. [Magnesium, strontium, and carbonate ions are also complexed into aragonite, and their importance in a reef aquarium cannot be overstated.] The rate at which calcium is extracted from the water is determined by the stocking density of reef-building livestock, type of lighting, and other biological, physical, and chemical conditions; therefore, each aquarium will have different requirements for the rate of calcium supplementation.