I have a school of 8 piranhas in my 6ft x 3ft tank. 240gal. I keep a pleco, jag, texan, oscar, convicts, salvini, 5 clown loaches and various catfish and an arowana all together. For quite a while too, and there have been no problems from anyone. Just keep lots of drift wood and plants for hiding or sleeping places and everything should be ok. Although some people do say you can't keep piranhas with anything.