I have owed Oscars for many years. I love Oscars because they are like big dumb, lazy dogs. I never saw them go after another fish (that wasn't a feeder) as long as they are well fed. They wouldn't go after anything that was quick, or was smart enough to hide. I was told I couldn't keep anything that could fit in their mouth in the tank. When i finally traded the fish in for turtles, i had 32 fish total, and the Oscars were about 12" long. Don't believe what everyone tells you. And as a side note. if you want their colors to pop, keep the tank away from windows, use a black background, and a black, if not dark substrate.
