Epsom (magnesium sulfate) salt is not a very good choice to solely supplement reefs with. It can be used in emergency, but if used continually, especially in higher demand tanks the buildup of sulfates can be detrimental to the tank. Your tank needs a mix of magnesium, chloride, and sulfate salts, so using a proper blend is the best choice. Kent Tech-M uses solely magnesium sulfate, and is therefore an over-priced and poor choice for supplementation. SeaChem Reef Advantage Magnesium uses magnesium, chloride, and sulfates in a balanced blend and is my choice for magnesium supplementation.
Take a read through the Guide in my signature, it explains the different options for supplementing, and my preferred brands.
