Because it's a predator fish. Some people say that it is not dangerous for the humans as in real life they don't atack us, other say the opposite, relying on the Holywood movies. The only sure thing is that piranhas eat meat from animals or other fish. I have piranhas in my aquarium. I bought them 2 month ago and since now, they didn't get into troubles. I do have and other fishes in the tank and as I was expecting - the piranhas don't atack them. The piranha is a beautiful fish, worth having it in the aquarium. Now I try to make his aquarium look more like his house, that's why I bought some live rocks, algae and marine plants.