Magnesium is present in NSW in a fairly high concentration (1300 - 1400 mg/L). Magnesium is an essential part of chlorophyll, which is necessary for photosynthesis. Without photosynthesis plants, including algae and the corals, which we usually have in our aquariums, would not be able to live.

Magnesium has another important function since in fact makes maintaining the correct combination of calcium concentration and alkalinity or carbonate hardness possible.

The explanation is as follows. Calcium forms with carbonates and bicarbonates an insoluble compound called calcium carbonate. Yes this is indeed an important building stone for corals and calcareous algae but then it should be formed by biological processes and be deposited at the right place. Therefore formation of calcium carbonate by chemical processes should be avoided.