i got into piranhas a few years ago.
let me tell you, they are over-rated.
if you're gonna get one cause you think they're bad ass, then you're wrong. not saying that you are though.
i first p's were a "purple" red spilo and a rhom, both at around 4".
they were very skittish when i got them and were for the the following six months or so. they wouldn't even eat in front of me for and would only eat when i am gone or the room is completely dark.
i got tired of it and then traded the spilo into a lfs for some red bellies cause i wanted something that would eat in front of me.
the red bellies were great, they were very active and skittish. but ate every shrimp i tossed in. i also thought they had more personality.
if you want a serra like a rhom, then i'd suggest you go to your lfs and pick one yourself that you like. i ordered mine online and i think that was one of my mistakes cause not all piranhas are going to have that fearless attitude.
if you do order one online though, you're gonna have to give it plenty of time to adjust and one day it'll come out of it's hole.