Here is some good info on the mag supplements
read all about it in this article
http://reefkeeping.com/issues/2006-07/rhf/index.php#14
here are two paragraphs extracted from that article..
Epsom salts (USP grade magnesium sulfate heptahydrate) is readily available in drug stores and is fairly inexpensive. The problem is that if this were used to raise magnesium by a large amount (or a small amount several times) the aquarium water would become enriched in sulfate. This enrichment may not be a problem for some aquaria, especially those using salt mixes already deficient in sulfate, or those that employ frequent water changes. Nevertheless, Epsom salts alone is not an ideal magnesium supplement.
A second DIY material is magnesium chloride. Some grades of magnesium chloride traditionally have been contaminated in ways that would preclude their use in aquarium applications. They sometimes contain
ammonia, for example. So any random magnesium chloride brand that's selected may not be acceptable. However, magnesium chloride hexahydrate from the Dead Sea Works seems to be adequately pure for this purpose. It is sold as a deicer (MAG flake from hardware stores, for example) or as a dust control agent for equestrian arenas.
I get my epsom salt from costco/kings/walgreens, and order my mag chloride from BRS to play it safe. Although I know some people have success using mag chloride products that are found in hardware stores called road-runner or safe step... I shy away from those since everytime I ask in the reef chemistry forum they claim those products contain higher impurites then the products from BRS.. There is a company out north that sells Magflake but will only sell it by the pallet. I had a thread on here a while ago about it. I think it was 20.00/bag and the pallet had 72 bags on it. Wasn't much interest in a group buy unfortunately.
