you could have one Serrasalmus sanchezi in there, these piranha's often lived on there own anyway because of there such aggressive nature. heres abit of info on them;
Scientific Name:
Serrasalmus sanchezi
Common Names:
Ruby-Red Piranha, Ruby-Throated Diamond Piranha
Distribution
Rio Itayi, Rio Pacaya, Rio Ucayali, Peru
Maximum fish size
7"(estimate)
Distinctive/unique features:
Red eyes.
Black terminal band on tail fin.
Aquarium:
Juveniles and sub-adults can be kept in a tank measuring 24x12". When they reach a size of 4-6", a 32" long, 12-14" deep tank is more suitable to provide enough space to move and turn freely. Adult fish need a tank of at least 40x16x16".
To shoal this species, at least a 52x20" tank is required: for each fish at least 25-30 gallons is required.
Temperture
24-30
Ph
5.5-7.5
Compatibility
This pecies has been shoaled in the past in a few rare instances, although they are not very tolerant toward thier own species. Due to the aggresive natue of this species most attempts are very short lived.....