it all depends on the fish, especially when it comes to Os. They have quite a wide range of temperaments and personalities. I put my aro in the tank first and got him settled in at 11-12". The Os were 5" at the time. It worked out for a while but after the Os settled in, they got more and more aggressive over food and feeding time. The aggression eventually extended past feeding time and became harrassment whenever they felt like it. I have also seen my same Os get along with a 2" bluegill with no problems. I just added a green sunfish and a 3" bluegill and they murdered them overnight. You never really know what will work. If you are going to try it, watch them closely and have another tank ready if it doesnt work out.
