thanks john,
i got this from gary
"Magnesium acts as an inhibitor to calcium carbonate precipitation. Magnesium loosely binds with carbonate and makes it unavailable to calcium to react with. This, in effect, provides a reservoir of carbonate and allows you to push your alkalinity and calcium concentrations higher without causing precipitation. Magnesium carbonate is much more soluble then calcium carbonate. As the carbonate concentration (alkalinity) drops, the carbonate ion is freed up from the magnesium complex and is made available to do the things that it does such as buffering pH and feeding coral growth.
How was that?
OK, now Richard's explanation. Calcium and alkalinity are like two different colored jelly beans in a jar. You can have so many of both color in the jar. When the jar overflows, you get precipitation. By keeping the magnesium level higher, you are basically increasing the size of the jar and allowing for more jelly beans to be added without causing precipitation.
Take your pick as to which explanation you prefer."