When you say phosphorus, are you sure you're not reading phosphates?
If your not, that's another reading you may want to be taking,,,
I have gone months with no growth, and then they start to a bit...
Also... I think someone asked, but how are they dying? Bleaching out must just be sps but what about the LPS youre talking about? going to mush and falling apart, just stop extending?
(P.s. I thought I was having a problem because ALL of my mushrooms and ricordias were dying... They were beating eaten by an evil Eunice worm I didn't know I had)