Very old thread here but I'm reading it so others will, pacu do not belong in a 500g tank either...
I was watching season 6 of river monsters and that piranha that bit through the metal cable, that is one dangerous set of jaws and I've seen rohms on bite pressure devices hit 80lb per square inch on another show, I believe they tested pacu too with a lower biting pressure. Lb for lb piranha is the king of fw fish biting Imo. I had a manny and it's the only fish I've been scared to put my hand in the tank with, he was insane... Had no problem with red Snakehead, mbu puffer, Cichlids, aro... Well a hairy puffer went for me once but I still put my hand in...