How low is too low with magnesium? I have recently been adding a lot more acropora to my 90 gallon and it seems like I have been doing very well with them overall. Over the summer I noticed them slow I g down as far as growth, after some reading and testing I found my alk was low and calcium was a bit too. So I have been taking care of that and keeping the alk up. Ninth past month I bought some really nice new acros that seemed to do well for about four weeks. Now two of the four new ones, about a week apart, have melted start to finish in less than 24 hours (3" frags looked fine with polyps out and good color) Tested the calcium was at 400, the alk was 8, the nitrates 0, phosphate .2, but now the mag was way down at 1200 So will low mag stress corals to the point of melting?