04-23-2009, 09:16 PM
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I still don't see anything unusual here. This is simple chemistry. Your fish release organic acids directly into the water. These acids are either removed by your protein skimmer, or biologically processed by the live rock. In this case, the acids interact with the buffer system, removing buffering ions. Magnesium is one of these ions.
The real question here is not why magnesium is falling. The question is why magnesium is falling at a pace that does not maintain the normal calcium to magnesium ratios found in natural seawater. I can only assume that you are adding a calcium supplement at the needed dosage, but are fixed on this recommendation for magnesium, which has been shown to not be enough to replenish the depletion in your particular system. Further increases to your magnesium additions are needed, beyond the recommended maximum dose.
By the way, 2 thoughts occur to me on this. First, I bet you have amazing coraline algae growth. Second, there is no such thing as a magnesium problem that does not involve calcium.-)