Do Acros use a lot of magnesium?

[Proteus]

I dose it as part of balling salts. But if I'm not mistaken Coraline will readily use it

[Reefer Rob]

I'm curious how you're dosing for alkalinity. If you're dosing sodium carbonate instead of sodium bicarbonate it could be precipitating out with calcium carbonate.

When I had a mature SPS reef I went through quite a bit of Mg and I auto dosed it. With only a few corals as long as you're testing and adjusting your water change water you shouldn't need to dose much. Instant Ocean has been ridiculously low in Mg lately.

I would consider 1300 to be the ideal level for Mg, but I don't think 1200 would hurt your corals at all. Some people successfully keep a reef tank without even testing for it.