Keeping large Pacu in tanks is a catch 22 situation. In a small tank they cant swim around and school so you miss out on some of their natural and playful behaviour. If they are in a largish tank however, they can get some momentum happening when they spook. A friend of mine once had some Pacu which burst the end off his glass tank, not from smashing into it (although they did do that frequently too) but simply from the hydrostatic force wave generated when one turned and swam away rapidly. He happened to watch the whole thing happen like slow motion and his story has stuck in my mind since it made such an impression on him.
I keep mine in a 3mx1mx1m acrylic tank and wince sometimes when they slam into the ends. I can feel the impact travel through air s well as hear it. I cover the end so they can see the boundary a little easier but I would never keep large fish in glass.