Need Some quick help, please

hi. I am a experienced fish owner but not a piranha owner. I want one mean piranha, I heard serrasulmus are the way to go? Well I don't want to get a huge tank for a solitary fish.Rhoms might get too big but they grow very slow.Whats one of the best solitary fish for a 55-90 gallon tank. I don't want to spend a lot for a fish either, like buy a fish that grows way too slow and have to buy it big in order to see its true aggression.Any imput would be great. I was thinking Rhom, brandti, and elongatus. Is there any other good mean solitary fish I don't know about? And Im not sure how fast an elongatus grows either? If you help me out it would be greatly appreciated.