+1 I agree with jtotheb. This is most likely the problem. Keep any melting zoas away from healthy ones. I also agree with using carbon whenever you have an issue like this. Good luck.Do a quick search on fungal and or bacterial infections. Maybe what you got. If this is whats going on you have to get on it right away. It can wipe every single zoa in a tank if its left untreated. I collect zoas and have had many outbreaks. Some outbreaks come on slow and some hit so quick and hard you will think your tank is about to crash. Good luck and I will follow thread and help if I can.