tank mates will eventully end up dinner. and an other nibble i dug up was also not to mix the species of Piranhas together as they will fight and not get along together. Some people have suggested JD oscars or Green Terrors as suitable tank mates I dont agree. I read also that a armoured plec was kept in a tank . Personnaly i would not keep any in the tank... I have not kept them would tyr if i had room.... these are my thoughts and reasonings form the info from the net... Its true they are specailize species to keep thou....Good luck