Robert,

I went through this same thing a while back. First off, like Josh said, don't dose things you can't test for. You really up your chances for a catastrophe. Salifert makes excellent test kits. IMO, all you need is a Calcium and Alkalinity kit and a pH probe, but go ahead and get the Magnesium kit to put your mind at ease.

Magnesium is usually not the problem. My problem was that I was not adding enough calcium. Recommended dosages are meant for maintaining calcium levels, not rising them. Once I kicked up the dosage, the calcium level came right up. I added magnesium for a while and I now realize I wasted a bunch of money on those additives for nothing. Your water changes are sufficient enough to maintain good magnesium levels. Hope this helps.