I dont think your thinking of the right gars, gars are a NA native and piranhas are SA natives. Also It would take an awfully large gar to eat a piranha. Gars are mainly minnow feeders. Only a gar bigger then the 55 could swallow a piranha as piranha are a tall fish like a bluegill. Gars are a peaceful fish and dont need the constant bother of piranha. If the piranha dont eat the gar then they will most likly stress it out until it dies.Amazon_Addict;3162826; said:Gars are known in the wild as piranha eaters. So it depends on the size of the gar and size of the tank. If it was like a 700gal+ tank then they would probably be just fine, at least for awhile.