Calcium is not the only substance needed to form the skeletal material of corals and allow calcareous algae to grow. Carbonate and bicarbonate are also needed and these two substances can have a major effect on stabilizing or buffering pH levels in the aquarium in the correct range of 8.1 to 8.4.
Magnesium is present in natural sea water in a fairly high concentration (1350 - 1500 mg/L) and is an essential element of chlorophyll which is necessary for photosynthesis. Without photosynthesis plants, algae and coral would not be able to survive.
In a properly cycled aquarium, nitrite is converted to nitrate and then, if there are areas deprived of oxygen such as in live rock and other filter materials, this nitrate is converted back to nitrite and then transformed into harmless nitrogen gas.
The pH value in a marine tank has a major impact on the growth of corals and calcareous algae and on many biological processes. The pH should not be allowed to fluctuate more than approx. 0.2 units during any 24 hour period and should preferably be kept above 8.2.
If phosphate is present in too high a concentration there may be an increase in growth of undesirable algae and if the phosphate level is higher than 0.04 mg/L the growth rate of corals and calcareous algae may decrease by as much as 90%.