Can you put piranha's in a 10 gallon tank?

Definately not in the plural form. A 10 gal tank in use only holds around 8 gals or so. 1 Piranha would be over maximum capacity.. The basic rules are. 1 inch of fish needs at least 1 gallon of water (a Piranha is about a 10 inch fish) Every tank must have a permanently running cycled filter. Every tank needs at least 50% of its water replaced every week.