naso tang death

[verlander]

I just lost a beautiful naso tang. It was healthy with a voracious appetite two days ago. Yesterday it would not eat, appeared in distress; its only symptom was discoloration (greenish) at the base of its torso. Its belly was as full as usual, maybe more so. There were no external signs of ich. This morning, it was dead. This tang had been thriving in my reef tank for several months. The rest of my tank is healthy and my chemistries are tuned. The only variable was the introduction last week of a purple firefish and a rabbit fish. Any ideas? Thanks.