Has anyone seen anything like this?

[Bob I]

I have just finished redoing my 33G semicube due to a strange occurrence. The tank has always housed two Purple Firefish, and a Bicolor Blenny. They always coexisted just fine until a couple of days ago when the blenny developed a hatred for one of the firefish (I don't know how he told them apart as they are the same size). The minute he spotted the object of his hatred he would chase it into the rockwork, and proceed to nip at it until all the fins were torn. The poor firefish spent its time cowering among the rocks. He never touched the other firefish. I have had to tear down the tank, and remove the blenny.