My 2 Red Belly Piranhas are kept in a 60 gallon acrylic tank, and my Wimple Piranha is kept in a 10 gallon glass tank (temporarily). The only tankmates I have for my Reds is a Bushy Nose Pleco. I used to keep a Sailfin pleco and a Red Line Pike Cichlid with them, but the Sailfin was killed by the Bushy Nose Pleco and the Cichlid became food for my Piranhas. The only tankmates with my Wimple Piranha is a Striped Raphael Catfish.

These are the following filters I use in my tanks:
60 gallon - 303 Fluval canister filter. The bottom module is filled with pre-filter media, the middle module is filled with ceramic biomax rings, and the top module has the 303 foam in it.

10 gallon - Aquaclear Mini with the foam filter block at the bottom. The rest of the media space is filled with small lava rocks for increased biological filtration.

Since Piranhas come from the greatest River in the world I try and provide very clean water for them, meaning lots of water changes. I change 10 to 15% of the water every other day in both my tanks. Some people may think this is too much, but I havn't encountered any problems, and the water quality is always perfect.

This is the current diet for my Red Belly Piranhas:
Dead food: trout, catfish, shrimp (without the head and tail) and freeze dried krill (occasionaly). Live food: goldfish, ghost shrimp, redworms and nightcrawlers. The following live food is what I have fed my piranhas in the past: baby mice, and waterdogs (salamander larvae).

This is the current diet for my Wimple Piranha:
Dead food: trout, catfish, and shrimp. Live food: guppies, ghost shrimp (he waits for them to molt, then eats them), and redworms.