RTN has many causes and the speed that the symptoms show up can be shocking to a reef-keeper. I have a PC-rainbow acro that showed up with patches of tissue necrosis. There was no alk or temperature swings, but I fragged my PC sunset milli the day before. The rainbow acro was nettled or stung by the milli after fragging and this caused the loss of tissue.
From your photo, there appears to be some algae which is growing adjacent to this coral. Some algae release toxins that can kill coral tissue. Earlier this summer, I had an outbreak of hair algae which killed a few acros. Coral death was caused by the toxins produced by the algae. Acropora that were not in contact by the alga were not affected. 0.04 for your phosphate seems OK, but there appears to be more available PO4 for the algae to thrive. Your phosphate may actually be higher than the 0.04 value from your test. Also, keeping the lights off for a few days will alter pH and alkalinity due to CO2 production without the presence of photosynthesis. And that will cause an alk swing downwards and overcompensation caused the alk to rise suddenly. You also have a filefish in your tank and these can nip and feed on coral polyps when hungry. Just some suggestions for you to think about and I hope that your coral recovers............Jim
