It helps keep alk and calcium at higher levels.. usually when people report that theyve been adding calcium and it never elevates, it always has to do with the magnesium, unless test kit is bad. Usually people keep magnesium in the 1280 to 1300 range. You cant really lower levels unless they get depleted by your reef. I would only worry about magnesium if youre having problems with alk and calcium.. it's not dosed often as calcium or alk, but it is important

You can make your own, which is the cheapest, or you can buy some that are already made by Kent which is more expensive.

Elos is another good test kit, but it only tests by 100s.. like from 1200 to 1300, which isnt bad.