Problem With Chromis

[super7]

Hey I have four blue green chromis and one is really getting picked on. He has some missing finnage(nips here and there) and a gash along the top of his head. Wondering how to prevent infection and aggression. This is a reef tank that has been running for just under 2 months. The other fish are fine though, same with the snails,toadstool and zoos.

Thanks Super7