Same thing happened to me with PO4. I was purposely dosing to fight off dinos and added too much and I had above 2.5! At first corals colored up and then a week later they started to brown. I also noticed that growth halted to a snails pace. GFO didn't help me at all but the liquid PO4 removers did. I now am at 0.2 and still see the need to lower it. I agree that acros are not happy at high PO4 levels.