Rapid Tissue Necrosis is just the degeneration of tissue, oftentimes sloughing from the skeleton and the waste produced triggering other skeletal coral (mainly sps
, but also lps
) to follow suit. It is a big deal because it is a very fast-acting "disease" that I consider highly contagious and there is no singular causative agent (please correct me if I am wrong). Just from experience I have seen this more with actual water stability problems occuring and, in general, is a commonly encountered result across the board in terms of negative responses (even fragging). I would test Salinity, Calcium, Alkalinity, Magnesium, and pH just for kicks to see if anything has bottomed out...doesn't hurt. Other than that, all I can advise is not to keep soft corals and hard corals together in a frag tank.