Piranhas are illegal in many countries/states because of irresponsible owners releasing them into local waters. When any fish (espically piranhas) are introduced into un-native waters they have the potential to create havoc in the ecosystem. Wheter it be through spreading of disease, preying of fish, destruction of native plants, fish eggs/nests being destroyed, and many many other problems. What worries fish and game authorities most about piranhas is the possibility of them killing native fish. A decent size shoal of red belly piranhas could wipe out a good sized lake of all fish and other prey throwing that whole ecosystem out of whack. If the climate is warm enough for the piranhas to live over winter, they could even breed. There are actually many species of piranha thriving and breeding in Florida ponds, rivers, and lakes. Those pond's, river's, and lake's native fish are almost completely gone, which is an absolute shame caused by irresponsible owners.