I've been using Fritz RPM for years, and while I like the the salt, I have always had to keep an eye on Mg levels. When I first started using it my Mg steadily climbed after each water change until it was over 1600. I contacted Fritz who insisted it must be user error with the test, but I was running samples from multiple tanks at once and getting consistent results. By the time I figured out it was the salt I was using a box from a different lot and Mg levels had started to drop. I've also had problems with low Mg. RPM is usually reliable, but every so often I get a batch that has high or low Mg. Calcium and alkalinity have always been fine.