Water current is a very important component in the growth potential recipe for piranha. Introducing water in the piranha tank stimulates their metabolism, which in turn stimulates the immune system.
Here we are giving the fish a work-out. In a captive environment and having an owner who will feed you what ever amount you can eat, really can get the fish into a very lethargic state. The result can be; lowered immunity, fat, and stunted growth with an increase in aggression, nervousness, and disease.
One of the solutions is quite straight forward and inexpensive. We can use a powerhead to get the water moving in the tank.