Some ask "What is RTN?" It is an acronym for Rapid Tissue Necrosis, which means the tissue is dying fast. Ever heard of 'flesh eating disease' on shows like ER? It has to be cut off quickly before it gets into your entire body. Acropora suffer from RTN, and it just happens seemingly for no reason whatsoever.
Here's a lovely piece.
And then a week later I saw this:
Basically, the tissue begins to look very thin, and just sloughs off the coral skeleton in flakes or disintegrates. Here are a series of pictures of it, taken out of the water. I removed it so it could be fragged, cutting off all the healthy pieces in hopes part of it might live. While out of the water, I took a series of macro shots just in case there were parasites I was overlooking.
Here's the core, 24 hours later.
Sadly, the fragged sections didn't give me much hope.
I hope these pictures help others in identifying this situation in their tanks.