I've heard of people flushing dead goldfish down the loo, but not 14-inch piranhas!
These fish do have a scary reputation, but a lot of it is hype. I had to do research about them when I worked at a rainforest centre, and in the wild they largely feed on carrion and detritus which falls into rivers - in fact they play an important role in helping to maintain water quality. They're only considered dangerous if a large number become trapped in a small pool (e.g. when rivers dry up in the dry season) and food is in short supply; then if an injured animal was unlucky enough to blunder into the pool they would certainly polish it off.
When I was in Bolivia about 10 years ago I was wading in a stream in the Amazonian rainforest (trying to cool down), with a number of other people. The Bolivian lady who cooked for us was fishing in the same stream, catching our supper: when I asked her what fish she was after, she replied "Piranha" and showed me some she'd already caught! I did leave the water fairly soon afterwards, but neither I nor anyone else suffered so much as a nibble. In fact, rather than me being eaten by piranha, I actually ate them for supper... And quite nice they tasted, too.