Are you sure what you are dealing with is even tissue necrosis? I noticed you kept using the term bleaching for the way the corals look, and with RTN and STN the flesh of the coral actually falls off the skeleton. I've lost a few acros from RTN and with me the coral was nothing but skeleton within a day (probably within a few hours honestly). I don't know what effect your really high nitrate and phosphate levels would have on sps corals, but the description doesn't sound like tissue necrosis to me. Maybe some pics of what the coral looks like.
