I have a couple 2" red belly pirahnas in my 75 gal. They're great fish to keep. Just remember that they grow to 12" so you'll need to get a large tank if you plan on keeping them for a couple years. I think you can put 3-4 full grown in a 75 gal. They grow approx. 1" per month for the first 6 months and about 1" per year thereafter, or so I've been told. They are wonderful fish to watch and I really enjoy having them in my tank.
Major drawback is the fact that they severely limit what else I can put in my tank. I've provided plenty of room and hiding places for them and they continue (of course) to hunt down most everything else in the tank. I've currently got a pleco, which they don't mess with because he's just a turd factory and probably doesn't taste too good. I'm down to 1 cory, they finally caught the other one when he spazzed and forgot where his hiding spot was. Three crabs seem to hold there ground just fine in there as well. Anytime a pirahna comes within a few inches, they just throw the big claw up and the pirahna backs off. Any schooling fish, i.e. zebra danios, will be hounded constantly by aggressive fish like pirahnas. I put 9 zebras in the tank the other day and within ten seconds I lost one. Within 24 hrs, I lost 4. After a while the pirahnas get a little tired of chasing the fast zebras and they just hunker down and wait for me to drop some dinner (fresh shrimp) in. Overall, great fish. I still would like to add some color to the tank and I can't do that with the red-bellys in there.