Reef Chemistry 101 Calcium – CA Calcium is a crucial element on the natural reef, and in fact is a building block of the reef itself. Many corals, as well as invertebrates, clams, desirable algaes, and other creatures on the reef depend on adequate calcium being available to them in the water. PH PH is simply a measure of the relative balance of hydroxide and hydrogen ions in water. In layman's terms, this means that there are hydroxide elements and hydrogen elements