I have two oscars that have been together for many months and have got along fine, but now sometimes they fight really aggresivley. i dont think its just play fighting because one will charge the other from the other side of the tank and ram it hard, and they will go for each others gills and make numberous attempts. however, sometimes they still swim together side by side. what makes matters worse is i have 2 red devils who seem to have taken sides with my albino, and sometimes all 3 fish will gang up on my red oscar(who is my favorite fish) and bully him into a corner. the red devils dont attack him as ferociously as the albino does though, they will mostly chase and nip and try to bully.