I doubt the oscars are retaliating, but hey who knows right? I think you are overstocked, but that is only because I like to give the silvers a whole lot of space (250 gallons plus) due to their size. If the aro is at one foot and the oscars are pushing 10 inches, I would not be suprised at all if the agression (and eating in this case) is due in part to space constraints. That said, I can understand holding off until you get the bigger tank- In the meantime, I guess there is not much you can do short of dividing the tank or moving some of the fish. The arowana is going to eat anything smaller than 6" if it is feeling either hungry or intimidated. The oscars are big enough to be safe I suppose, and even seem to be the bullies. Do you think your LFS would hold either the arowana or oscars until you got the new joint set up? For a small fee, of course...