No piranha can be kept in a 29G permanently.
You can keep a few (less than 5) exodons in a 29G. They are scale eaters just like piranha, and actually much more aggressive than piranha. But be adviced exodons are difficult to upkeep, most will kill each other, especially in small shoals. Good idea to have a lot of plants or hiding places in the tank.
You can buy exodon from here:
http://aquascapeonline.com/prodView.asp?idproduct=36
You can keep a few (less than 5) exodons in a 29G. They are scale eaters just like piranha, and actually much more aggressive than piranha. But be adviced exodons are difficult to upkeep, most will kill each other, especially in small shoals. Good idea to have a lot of plants or hiding places in the tank.
You can buy exodon from here:
http://aquascapeonline.com/prodView.asp?idproduct=36