Its a difficult task owning Pygocentrus Piranha because you could have a nice mellow tank for months and P's doing well, no signs of aggression, then next morning you turn the lights on and a head of a £100 Piraya is sitting at the bottom of your tank.
Unless you have a good supply of cheap P's like RBP's, its always tuff unless you have a monster tank where they can hide if hit.
I sold a lot of 9"+ Piraya, big Caribe and 10"+ Reds to a guy in London, but his tank was 660 UK Gallons with 8 powerheads in it. Amazing site and he never lost a fish due to the space they had.
Nickg