Calcium alone cannot form the skeletal material of corals and allow calcareous algae to grow. Some other substances are needed as well. A few other constituents are carbonate and bicarbonate.
These two substances also have a major impact on the stabilization of the pH in the proper range of 8.1 — 8.4. Such stabilization is also called buffering. The total carbonate and bicarbonate concentration is also called carbonate alkalinity or carbonate hardness.