Catherine, if i were you I think I would leave the tank alone for a while (as hard as that is to do).
I don't know what fish you have but minimal feedings, no water changes (just top off), dose a 2 part as needed, keep your hands out of the tank
,keep it very simple and let the tank age and settle a bit.
Low Mg will inhibit coralline algae growth but i don't think it would make your sps die
Of course you might not want to listen to my advice my tank isn't doing great right now
*edit*Looking at your parameters I'm wondering if maybe you aren't getting a bit of precipitation and that is why the mg is so low. It might be worthwhile to take the calcium reactor and kalk off line and find out exactly how much alk and cal you are actually using in a day