I have 2 oscars in a 90 gallon in wall tank, no other fish, I tried a pleco and one of the oscars attacked, killed and ate the pleco, actually ate two of them. my question is these two oscars especially one of them is brutal to the other, they are not crowed, well fed, any idea how to get them to get along, they have been together for several months but the fighting is getting worse, one is about 6 inches the other 5 inches