I added a Schooling Bannerfish to my tank (180 gallon) on Feb 11 from Dr. Reef along with a snowflake eel they were the first fish I added. He's always been very social, swimming around the tank, following anyone who walks by. On Feb 21 I moved a blue spotted puffer from my 40G tank to the 180G. On March 2 I noticed the dorsal fin on the bannerfish was nipped off. I'm not sure if it was the puffer or the eel, I've never seen aggression from either, the puffer and bannerfish regularly swim together and I usually on see the eel during feeding time. I'm feeding the eel silversides every other day. On March 5 I added 3 more fish from Dr. Reef, a foxface, yellow goatfish, and a big eye black bar soldierfish.
Today I found the bannerfish at the bottom of the tank, he's moving around a little bit but not a lot. Other than the damage to the dorsal fin I don't see any other damage on him.
None of the other fish appear to be having any issues.
Salinity is 1.025, Nitrate is 8.6, Phosphate 0.17.

I feed them multiple times a day, and they quickly consume it all; like Jay said, sometimes I'll add to it almost immediately. (Of course, not enough to float to the bottom/go down the weir and foul the water.) But I am sorry for your loss!