Gilbert,
Magnesium is not terribly important other than it interfere with the crystallization of CaCO3 thus allow us to keep Ca and Carbonate to be higher than saturated level in salt water. Natural sea water also has supersaturated level of Ca and Carbonate also not just in our aquarium.
This was how it was explained to me:
Mg precipitated on CaCO3 surface and coated it. This action prevent Ca and Carbonate ions form crystallizes on this surface thus get removed from the water. Without Mg interferences, Ca and Carbonate will crystallize out of solution at a much lower level.