I've just started using Tailored Aquatics Calcium Plus and Reef Magnesium. The Calcium Plus contains calcium, magnesium, and strontium (in a tiny amount), along with organic carbon... not ideal for my setup (skimmerless) but it will have to do... I like that the Tailored Aquatics products are made in Canada
Anyhow, it sounds like you need to do some reading on the relationship between Calcium, alkalinity, and Magnesium...
http://www.reefkeeping.com/issues/2006-06/rhf/index.php This should get you started. Next will be test kits... as a few people have already stated, dosing without testing is dangerous... you don't want to lose a tank of expensive fishes or corals to an overdose.