heres my dilema: I want piranhas that are really aggressive, ones that will eat goldfish and mice like theyre starvin etheopians, ones that will try to bite my dick off when i walk by the tank, but ones that arent aggressive to each other. How do i reduce aggression amongst the ps while maintaining their bloodthirsty behavior? I was thinking that a powerhead might keep them occupied, so I have some questions. Where should I put it? Near the top or the bottom? I dont want the powerhead to be on them all the time, because then they will just hang at the opposite end of the tank and be so crowded that they will eat eachother. Also, what gph powerhead should i get for my 6 foot long (140 gallon) tank?