In need of help from experienced sps keepers!

[Aquattro]

My 180, full of SPS, ran at 1050ppm Mg for it's entire existence. I didn't have any issues with corals. The Mg value here is not the problem.

I also started my new tank with dead/dry rock, new water, new sand and 60 SPS frags. All did well at the same values OP has posted, and color and tissue stayed on the pieces. Higher PO4 eventually retarded growth, but never lost any tissue/color on a brand new setup with everything dry. Again, while not optimal in my opinion, not the problem here.