I would be inclined to agree with jonny though about getting a serrasalmus, possibly a manueli.. very nice fish!
I wouldnt recommend a mannie for a first piranha. Their sensitive to water chemistry and quality. Plus all red bellies have the potential to grow to a foot long with the right care, even tank bred's although they usually max around 10".
Look into serra's such as sanchezi, compressus, altuvei, all good little p's that dont require huge tanks. What are the measurements of your tank? a 125 high is no good, piranhas require a standard rectangular tank as it provides a bigger footprint (swimming area)