What can influence a tank's Mg reading

[Myka]

Salifert starts at something like 800 or...? I don't have it in front of me, but I'm SO sure that it has very small increments. Very strange that one tank isn't increasing Mg. Cal and Mg both interact with eachother though, so I would check your other parameters. Heck, i'd check all the other params...I do that anytime something seems funny and I usually find something else is off too.