Piranha get a bad wrap. Piranha are characins, big tetras. They are schooling fish. When you keep one in a tank by itself, it just sits. Two will usually fight. When you keep a group of them together, it is a site to behold. I have not been to the Baltimore Aquarium for a few years. But as you walk up into the rainforest, they have tanks that sit right on the floor. One of these tanks is full of red belly piranhas. It has always been my favorite tank at Baltimore. When I had my fish shop, I had a 180 gallon tank with 6 wild Pygocentrus / Serrasalmus maculatus, the gold piranha in it. When the sun would come in the front windows and shine on them they were stunning. So if you choose piranha's, buy 6 - 8 little ones and when they get bigger they will swim constantly.