a 220 gallon. shoot id go with 15-20 juvi rbp's or caribes if u can afford it. keeping 6 piranha's or less is one thing but if u ever seen a large tank with 20 or more with the right water temp and set up... thats when u see the true colors of the piranha imo. in nature they never roll in a pack of 6 lol. they're rollin 50-100 deep. hahaha
or get you're self a elongatus. you can't go wrong. mine is freakin BEAST.