Speaking from a lot of exprience and a lot of research when I was trying to deal with it, there is no real known cause for RTN, and it is seperate from bleaching, which is commonly caused by warmer temps, and sometimes lighting if the coral is not properly acclimated. Bleachng is when a coral expels its zooxanthellae, RTN is when the tissue of a coral receeds at a very fast pace, sometimes a coral can be lost within hours.The most common cause could be stress, in the form of poor lighting, poor current, injury or being attacked by a near by coral. To solove my problem I incresed the flow in my tank quite a bit, and went back to reagular additions of iodine, whether it helps I cant tell you for sure,but I havent had an outbreak, knock on wood, in well over 8 months, compared to having outbreaks every couple of weeks in the past. I havent had much luck treating corals with rtn with an iodine dip, about half the time the coral would die as a result of the dip, or would not recover, Ive had the most luck with dips when dealing with a known bacterial infection. If its a really severe case fragging works very well at stoping the recession. HTH
