After keeping a red belly for 10 years when I was younger, it was only natural that after buying another fish tank somewhat 10 years after the first Piranha died, that I own another one. Oh no, not me. I buy 4, but the difference is unreal. The single specimen was timid and would hardly eat anything, even being 30 cm long. The new four are about 5 cm and they even go for the dog as she walks past the tank. These fish are cool! :-)
I have four red bellies in my tank and I have found them really easy to keep. I feed them a mixture of goldfish liver and cheap steak. They have grown at least five times as big as when I bought them and haven't had any sickness.
I have 16 of these wonderful fish in my 6400 liter tank. All of them are about 20 centimeters long and are very agressive. I have been bit by them a lot of times while cleaning the tank. I wouldn't recommend this fish to a child. They are very dangerous, even to me. I also own a fish store which specializes in Piranhas. They overall are a wondeful and hardy fish to have and are fun to watch them eat!
The neatest display of RBP's I've ever seen was at the New York State Aquarium on Coney Island. They had at least thirty to forty full grown Piranhas in a huge tank...with Neons! Yes, about a million, billion Neons were the tankmates. I was talking with the Curator about this, and he told me that the Neons were so small, it wasn't worth the Piranha's time to chase them (that and the fish were quite well fed and didn't have to worry about a meal). Piranhas are illegal in NY, any confiscated specimens end up in the NY Aquarium. Piranhas are actually vegetarian like their Pacu cousins, and only opportunistic on meats, especially in the dry season of their Amazonian homeland when the rivers no longer flood the forest, causing a less abundance of nuts and fruits to dine on. Those teeth are more nutcrackers than flesh rippers. Those that feed viciously for the enjoyment of their owners are not properly cared for Piranhas, and one that has been raised on flesh may be difficult or next to impossible to wean over to a primarily veggie or Cichlid pellet diet, with flesh given occassionally as a treat (I like using squid or shrimp on a plastic fork tied to a broom handle). Most of all, respect that fish! They have been known to attack through the glass of their tanks, and not many tanks manufactured can withstand the rush of a Piranha.
You should NOT feed your piranhas lettuce and cucumbers etc. They need a varied diet, and minnows are better then goldfish, they do not contain as much fat as the feeder goldfish do. Beefheart is a good treat for them if you can obtain it. These piranhas - pygocentrus nattereri (recently reclassified) -are very skittish and can be scared very easily, it is a very BAD idea to be moving the piranha between tanks for cleaning, the nets can cause abrasion to the eye and permanent damage. Piranhas are one of the most abused fish in home aquariums in the world, only beaten by goldfish in my opinion. The average fish keeper should not own a piranha. The general rule of thumb is 3 liters of water for every centimeter of piranha, thus if you have a 25 cm Red Belly Piranha it should have at least a 75 liter tank (although the standard American 20 gallon tanks are unsuitable for a 25 cm fish, it is difficult for them to turn around due to the narrowness of these tanks.
I have been studying these fish for some 10 years now and can tell you all with certainty that these are the most interesting fish you can buy in a store today. I also have a very large red oscar in a separate tank, and have been current with him and other oscars for many years. The personality in the oscar is about as far as you can get, and the size is also impressive. But the red belly, white, and golden piranha are indeed the finest fish I have seen yet. The original 4 I had were bought at about the size of silver dollar. They grew a bit slow, but within the first year of owning them they more than doubled their size. They became thick and darker. The spots faded, the teeth stuck out more, and the eyes became yellow. They would behave with more conviction and less fear the bigger they became. It seemed like they knew their own power almost. Cleaning became a problem when I would nudge up against them with the hose and they would simply wiggle in a mocking manner as if to say, try that again punk lol. I loved watching these fish, and soon bought a 350 liter tank for them. They layed down below at the bottom and calmly waited for me to feed them. Unlike the oscar, there was no buddy buddy relationship. It was either feeding time, or it wasn't. That's what I liked about these fish, they seemed to have an almost evil vibe about them. Yet they are caring of their young, they work as a pack with excellent efficiency, and they seem to always back down from one another. It's amazing to see a piranha go after a fish 3 times its size and then flee from an equal size piranha.