getsome type of RHOM, pygo shoals are pretty cool you could a mix of very small cariba and red bellys in there but you definately hae to get a bigger tank in about a year cause they grow fast. with a rohm you could get one around 5 - 7 inch and have plenty of room and never have a problem with lack of filtration.. you could get a brandetti or irritans they are just as cool, but rohms are awesome and eventually will grow larger. personaly if im going to have a decnt size tank for one fish then i want it to be a bigger fish..
i recently got rid of my RBs for a RHOM because its alot less maintenece to keep a tank with one fish clean (and RHOMs are awesome), my RBs would stink up the water even with a HOB and canister filter and bi weekly water changes.
ive had my brazilian rhom in the tank for two weeks now and hes starting to become more active every day, i removed most of the decoration from the tank so he cant hide all the time so hopefully it will get used to having people around the tank and a strong powerhead to keep the water moving so it will excersice, in turn increaseing metabolism, which ulitmately leads to what most of us keep piranhas for, FEEDING TIME !
summary: get a medium size RHOM, put a powerhead in the tank to keep it moving and growing and enjoy.