i have had Piranhas for years, they are my fav fish and I have learned alot about them from just studying their behavior with one another and their feeding habits in relation to water quality etc.. another observation i have made over the years is this one.. Piranha tankmates DO NOT EXIST! sooner or later what looks to be a tankmate ends up a snack.. They are genetically programmed to rule their domain and kill whatever they can, they can't help it, it's just the way they are.... Sorry but that's reality.. Besides, why would you want anything else in there unless it's food. haha