Piranha are super messy. You can't scape the tank. They are VERY nervous esp. in smaller groups. They can break tanks if they are badly startled. Beyond eating fish (which other Species do as well ( and the Teeth , they are just a big Tetra ( or Tetra cousin ).
Most people get bored with their Piranhas pretty fast. It's only a novelty for so long.
There is a $5,000 fine I believe for releasing non-natives. At least in my area. Don't ever do it, not even one. They kill native species and screw up entire areas.
The Aquariums and Zoos won't take them.
A few people do take lifelong care of these.
Monster Fish Keepers might be a good site to read.