From Creatures Wikia
Piranha (Viscious fiscious)creature and critter alike! It is a unique fish in that it is capable of surviving in both salt water and fresh water . If not contained in its own pond, an infestation of piranhas can be nearly impossible to exterminate -and delicate ecosystems like that of the Marine Terrarium may take a very long time to recover.
The piranha pond is located at the far right side of the Jungle Terrarium, with a trap door over it - the piranhas can be put to good use eliminating grendels if one is willing to create a contraption to lure the brutes in.
The piranha is also a real terran fish.