Can i keep pirhana fish

[Pfish]

My shop keeper said that I could keep a pirhana in my 20 gallons aquarium but I think I read somewhere that they must be kept in schools :?:

Can I keep a pirhana in my aquarium.

[rebelagent]

My uncle had a pirhana and kept it in a 20 gallon for awhile( he upgraded to a 40). The only time he put any other fish in there was for feeding purposes. Oh and he did have an algae eater in his take also. It never was bothered.