Speck,
I test for Magnesium. The concentration should be kept between 1300 and 1500 ppm, and you do need the proper Mg level to maintain proper calcium and alkalinity. It says as much on the Salifert test kit instructions. I find my Mg level drops quite frequently.
I use Reefpure Concentrated Magnesium Supplement. My system has a CA reactor and I have SPS and clams that show constant signs of growth. My CA stays between 480 and 500 ppm.