The only agression i had was when i added the smaller oscars. I added them when they were around 4inch and thoe other oscars was 8inch and he bugged them for about 4days. The only other aggression I had was with silver aro. None of the oscars like him. All my oscars were raised from 1inch with other fish. I would wit on introduing the oscars with the other fish. They seem to small he.she might eat them. I would think the 5" would be fine but u might wana try another fish around the same size as a "test" fish. My oscars actuall live with a goldfish in the tank. can u take any pix of the fish???