People do a mix, 5 cups Mag Cloride, and 3 cups of Mag Sulphate to a gallon of water.
check out:
http://www.bulkreefsupply.com/store/...aged-Materials
You can just add one or the other, but you are trying to not overdo your chlorine or sulphates, they are very abundant in the tank, so hard to do, but over time...
Randy's page has a graph showing what happens over time, can't find the link anymore, it's his recipie that people use.
Also... Mag Sulphate is Epsom salts
might be cheaper, you just don't know about how much "other" stuff is in there without lots of research.