I don't buy the phosphate reason. Phosphate at 0.3 doesn't cause tissue recession. In fact, I don't know that I have ever seen a tank that was killed or harmed by high phosphate. I am not convinced phosphate really matters at all unless it's 0 or you have a sterile tank that grows tons of algae because the algae has no competitors or predators in it. I know of some beautiful mixed or sps dominant tanks with phosphate numbers above 1 and weed-tier acro growth.
What is the freshwater source? RO/DI or tapwater? Is there something that could be leaching into your tank (maybe something that is corroding in your sump? Maybe a pump's magnet has been compromised?) If you are seeing corals die back there is certainly something else at work here other than a test kit result that isn't approved by the reef forums. Does the tissue recession happen to all corals? Have you tried moving them? Could it be simply too much light/not enough flow?
What is the freshwater source? RO/DI or tapwater? Is there something that could be leaching into your tank (maybe something that is corroding in your sump? Maybe a pump's magnet has been compromised?) If you are seeing corals die back there is certainly something else at work here other than a test kit result that isn't approved by the reef forums. Does the tissue recession happen to all corals? Have you tried moving them? Could it be simply too much light/not enough flow?