manuelis like strong current and excellent water parameters so make sure you buy the best filters you can afford. there is no such thing as over filtering a piranha tank. i keep my water at neutral PH 7.0...piranhas love that. ammonia and nitrites are highly toxic but nitrates not so much...again, as long as you have excellent water parameters you'll be good. you're better off with a longer tank...they like to swim back and forth, not up and down so height is not an issue but obviously you'll need a tank that wider for them to be able to turn around when they get big.