Hi,
Red-bellied piranhas are actually not too difficult to keep in terms of water quality. Being South American Characins, they prefer warm (23-27 deg. C), soft and slightly acidic water. But I have bred them in the harder more alkaline tap water of my area. They lived happily for many years.
Depending on how many fish you get, I would recommend a minimum 80 gallon tank for them. I had 3 in a 140 gallon tank, and IMO, I would have been comfortable adding only one more piranha to that, had I wanted to. Large pieces of drift wood should be added at the ends of the tank to prevent them from smashing into the glass when frightened, which is potentially fatal. Other 'solid' decorations should also be provided to offer shelter areas and reduce stress. Of course piranhas don't eat plants per se, but their teeth are so effective that they will nonetheless shred the plants quickly. Use floating plants if you want live plants in the tank. Their location generally protects them from the fish's teeth. Plastic plants are totally not recommended, since they will also be shredded, and could be eaten by the fish.
Water changes should be performed often. Due to their diet, they can 'foul' the water even quicker than many other fish their size.
In my experience, piranha do not need live food. I raised them mostly on fish fillets bought at the grocery store. Mostly I fed them salt water fish such as ocean white fish, cod, and such. I tried on a couple of occasions to feed them fresh water species such as trout, but they were absolutely uninterested in them. It is recommended to occasionally feed them vegetables, since they occasionally eat this in the wild. If your fish accept vegetable fare, peas, zucchini, and slices of apples can be offered once or twice a month.
Safety is of course a big concern with these fish, rightly. I have found the red-bellied piranha to be an especially cowardly fish, except at breeding time, when they will defend their nest vehemently. But don't let this fool you! Piranha have a strategy when frightened of continually opening and closing their mouths while they flee. Anything that gets between them and where they are going will quickly be shredded! Even when they become accustomed to you, they can still be dangerous in other ways. I had trouble feeding my piranhas because they became over eager, and would rush toward me while I lowered the food. They jumped right out of the water occasionally to grab the food. I had to be swift to avoid injury. I was also fortunate they didn't bust the glass cover of the tank and 'escape' onto the floor. They are just as dangerous out of the water as in.
While I generally frown on keeping them because of the "gore factor" (And I'm saying this in a CP discussion forum no less!(g)) they can be interesting, personable pets that recognize you and learn your habits. They will destroy any other fish you add to the tank, but beyond this, they are rather gentle in nature, and in fact can become boring to many people. If they still interest you, and you can meet their needs, then by all means get them. But be very careful anytime you go near the tank!
Take care!
Chris