My magnesium levels are on par along with my calcium at 500, hardness 10 or so and now my phosphates are almost at zero, less than .25 and now.
Hi: glad to hear your coral is recovering now. In regards to your readings, I just wanted to clarify:
1) Mg should be approx 3x the level of your Ca (i.e. if Ca is 400, then Mg should be roughly 1200)
2) 500 is a bit on the high side for Ca, but probably ok (I think seawater ranges from 380-450 for Ca).
Based on the above, I normally target ~450 for Ca, which puts my Mg target at ~1350.
IME, Mg is something that I test for about once every 2 months or so, and I will normally find that I need to add Mg supplement to bring it back in line. (Ca & Alk I dose on a daily basis).
HTH,
Rob