Piranha

Sorry buddy, but that question is just too blunt.

First of all, what size of a tank is it, what kind of piranha is it, how big is he, and what do you have in the tank?
 
Easy answer is:

The piranha will likely consider any tankmate to be food sooner or later. Some have had luck with nocturnal catfish, bigger plecos, and exodons (bucktooth tetras). I'd avoid exodons with a baby piranha, they'll make a snack of him for sure.

That RBP could grow an inch per month or more, so a new tank will be in order soon.
 
Well...I hate to have to be the one to tell you this but that tank is waaay too small for a red belly piranha.
 
You probably need at least triple the size of what you have now. And soon feeder fish will be costing you every week. You might start a tank with feeder fish and try to breed them. Piranha have quite an appetite.

You best luck is a big pleco. They have a hard shell around them but thier bellies are soft. If they manage to flip him over, then he doesn't have a chance.
 
AquariaCentral.com