I would go with 2 AC 110's (500's) on the back and a good quality canister, Ehiem if you want to spend the money if not go with a fluval there not bad either. This should keep the water clean as I have heard that Piranhas are quite the messy eaters.
Agreed
Check out www.bigalsonline.com they have good prices also www.petsmart.com has a good price on them as well and if you bring the page into a store they will match the online price.