According to what I read here on MFK, they are nasty. I purchased 2 of them at a length of about an inch. They were aggressive only with each other and I quickly separated them and eventually gave one away. The other one never took a swipe at another fish while I was watching, and never showed any signs of having been hammered by anybody else either. He lived with a community of Oscars, a RT Cat, a Tiger Shovelnose, a Red Devil and several others; he was the only fish in the tank that didn't go nuts for feeder fish, either live or frozen.
I eventually moved him into a 360-gallon tank with a large number of small barbs, livebearers, etc. and I neer saw him even seem to notice them. Very peaceful and impressive display. I left him behind with a friend in Ontario when I left the province. If...big "if"...I ever set up a tank for a giant fish again, Osphronemus is at the top of the list for me.