Sounds like you put an eel in with small fish. They can and will eat anything they can fit in they're mouth and they think their mouths are bigger than they are.
Best thing to do with a big eel is give it lots of small damsels they are cheap enough that if the eel get them your not heart broken but and the same time they are small enough and fast enough that the eel will have a hard time getting them for food.
Also eels should be the last fish you put in just due to them being just about blind they establish who is all ready a tank mate and who is food by tagging the fish after they are introduced. I have done many nice reef systems with dragon moray and tangs, goby, and anthias with no loss due to the eel.
People not feeding the eel is not the eels fault for eating a fish
