Slow Tissue Narcosis(I think that's the spelling), Rapid Tissue Narcosis.
Basically the tissue of the coral recedes Slowly or Rapidly.
Not sure whatt the true cause is though. I have some corals right now that are STN'ing a little. I haven't figured out why yet, but been looking. I think it all started with a power head that fell and was pointed directly at one of them. It started to STN and the other's have just followed along with them. My Alk got a little high after I must of bumped the effluent rate of the CAL Reactor also. With that down now I would think it would have stopped, but I still noticing some tissue loss on a few. What's weird in my case is some of them have tissue loss and others don't. What ever my cause it's been going on for now approx 2-3 months.
Still trying to figure it out.