From RANDY HOLMES-FARLEY, Ph.D.
http://www.advancedaquarist.com/issues/oct2003/chem.htmMagnesium in Marine Aquaria
Magnesium has tremendous biological and chemical relevance to reef aquaria. Fortunately for reefkeepers, it is present in abundance in seawater. There is, in fact, a fairly high turnover of magnesium in reef aquaria with rapidly calcifying organisms. The primary reason that magnesium is not more of a daily concern to aquarists is that the reservoir of magnesium in seawater is very large. Magnesium might be compared to a large lake, with the lake level only slowly responding to changes in inputs from rivers and export via evaporation and the outlet. Consequently, maintenance of magnesium levels is not typically a rapidly developing problem. If using an appropriate salt mix, it may never become a problem for many aquarists.
Meaning that magnesium is normally not a thing for any of use to be concerned with.
The other major source is calcium supplements. Many of these supplements contain magnesium, either by “accident” (as in the case of calcium carbonate with impurities of magnesium carbonate that is used in CaCO3/CO2 reactors) or because magnesium is intentionally added by manufacturers.
Then if you perform recommended water changes, these levels will balance out, unless you use a bad salt mix.
Another potential source of magnesium is fish food. Magnesium is present in many such foods at fairly high concentrations, but not enough to have a significant impact on typical levels of magnesium (~1285 ppm).
This means that even when you feed your aquarium large amounts of food that contains large amounts of magnesium, this will not impact the levels in your system. Probable only adding organic and inorganic phosphates and increasing nitrates would be you concern here.
A 100-gallon aquarium contains about a pound of magnesium! In order to raise that same aquarium by 200 ppm of magnesium, one would need to add on the order of 2 pounds of dry magnesium salts!
Which shows that the increasing or decreasing of magnesium is a very slow process and it would take a sever change to the chemistry to the system to get it to change.
Randy then concludes with the idea that aquarium keepers are probable more concerned with increased magnesium levels then depleted levels.
Conclusions
Magnesium is an important ion for reef aquarists. In addition to its many biological functions, it serves to prevent the excessive precipitation of calcium carbonate from both seawater and aquarium water. Since both calcium and alkalinity are very important to organisms that we keep, making sure that they are not lost to excessive precipitation is an important part of aquarium husbandry.
It is my belief that any on that has problems with magnesium in their systems is directly related to their lack of good reef tank husbandry. (Water Changes)
Remember the old saying, "The solution to pollution is dilution"? The same is true when one does good water changes using RO/DI and good saltwater mixes. Then one would never have to worry with playing scientist with chemicals other then calcium/alk.
