those are NOT Pygocentrus Nattereri~!!!!
Correct me if I'm wrong but I don't think I am, those are actually Pygocentrus Piraya.
That would be the largest shoal of piraya in captivity that I know of. That's a lot of cash in that exhibit.
They even have a piraya picture on the information board and calling it red belly piranha (pygocentrus nattereri). that's false advertisement and provide wrong information to the general public. Next time someone go visit should point it out to their exhibit manager.