BGK vs Oscar

Question
I have a 240 litre tank with 2 oscars, 1 BGK, 2 convicts, 1 plecostamus, 2 bala sharks, 1 clown loach, 2 gouramis and 3 (different kinds) catfish.  Recently the BGK started picking on one of the oscars.  Really picking, like nibbling.  I see now that there is a little white patch where the BGK nibbled. I took the BGK out and put him in my other tank.  My question now is, will the oscar get better on it's own or should I medicate the tank?  The other thing I want to Know is why the BGK did this.  They have been together from the beginning (about 18 months) and there has never been a problem before.  The BGK is also normally not aggresive, although I am not sure this is aggresive behaviour.  Maybe the BGK is trying to 'clean' the oscar?

Answer
Hi Sam:  not sure what a BGK is but probably it was a territorial issue and you should probably not keep the BKG with the Oscars.  If BKG is black ghost knife... they can be aggressive....but whatever the issue is... they should remain separate.  If the Oscar is wounded badly treat him with Malafix...if not just add aquarium salt per its instructional label.