I have a 7" Pacu in with my 6.5" Reds and it's fine. Actually the stupid thing is pretty vicious. It's always trying to steal territory from the Reds. It does try to mimic them. I'm selling it today though because it's getting too big too fast. Make sure you get a jumbo tank if you're getting a Pacu cause they don't stop growing till they're like 2 feet long and over 20lbs. Seriously.