Alkalinity

Alkalinity is important, more so for SPS corals, because it indicates the buffering capacity of the water. The greater the buffering capacity, the more stable the pH.

And with a greater buffering capacity, you should also have more of the minerals available to build the stony skeletons and that SPS and LPS make. Compounds such as Magnesium carbonate should be present in relatively high concentrations so that the growth of stoney corals is not limited.

Carbonates help to provide the buffering capabilities of water, by binding with major ions like Magnesium, Strontium, and Calcium. These compounds form the basis for coral skeleton growth.