PO4 would probably be the main problem since it was so high (what kits are you using btw?). Since you're using Radions and 400 watt halides, I would suspect the corals are getting way more light than they would like (your lights are more suited to SPS), but they would look bleached if they were getting too much light. So I think the PO4 is probably the culprit given the information you've provided. You could use bumping your calcium up over 400 ppm too since that would balance better with the alkalinity level you're keeping.
As a side note, I had three brain corals (a Favia, a Symph, and a Lobo) slowly recede over 2 years to the point they faded away and died. They didn't do this all at the same time, and they were all well established corals. The Favia was almost 10" across, it was the first one to start receding and was the last one to completely die. I had that coral for over 4 years. Nothing I did would stop the slow recession. The tank was chock full of other brain corals that were doing really well, so it made no sense. All my parameters were spot on too. I even moved the affected corals to a quarantine tank and tried a variety of dips and treatments over 6-12 months. I couldn't stop the recession. Besides the receding edges, the corals puffed up nicely, had great colors, and otherwise looked healthy. I put it down to an unknown disease...possibly Band Disease.
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~ Mindy
SPS fanatic.
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