The only thing I can think of is that you're having a mini-cycle (well .. maybe not even all that "mini"). Compounding this is that the die-off from the cycle perpetuates that much more. If that's the case, the fish may likely weather the storm but who knows how bad the corals will do. The key is to get the tank stabilized. I don't know if these things work but everytime I have a tank disruption I dump some Cycle in. Whether it does anything or not I can't say, but if nothing else I get the feeling that I've "done something about it." Maybe some Reef-resh or Probidio or something that infuses and feeds bacterial growth might not hurt. I don't know.
Best of luck.
