These guys are very aggressive, and the only way you can calm them down is to provide more space. They are very territorial and will take on even the largest and most aggressive of fish.
Be warned: these guys can grow up to four feet or more. Make sure you can provide adequate space as he grows. They will need an extremely large aquarium.
We have one in our store--it thoroughly kicks the crap out of our large oscars (bigger than he is), severums, and pacu. The only fish he doesn't seem to pick on are the plecos. He has seriously injured quite a few fish and he's fairly isolated now.
But in spite of all that, he's my favorite fish in the place !