Eating at my clown

[ll206]

Hey guys. I'm pretty new at this hobby, so bear with me. I've had my tank up and running for about 5 months now; with no problems. One day I noticed a hole in one of my maroon clowns lower fins. With every day that passes, that hole seems to get bigger and bigger. What could it be? Its tank mates are a watchman goby, an ocillerus clown, a cowfish, and a fire shrimp. Help please

 

[craigj]

I'm not familiar with the ocillerus clown, but I have often heard that clowns rarely get along in a small aquarium. Unless of course they're pairs.