Heres tonights optional reading.
Do-It-Yourself Magnesium Supplements for the Reef Aquarium
http://reefkeeping.com/issues/2006-07/rhf/index.php
Magnesium in Reef Aquaria
http://www.advancedaquarist.com/issues/oct2003/chem.htm
A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH
http://reefkeeping.com/issues/2006-06/rhf/index.php