Yep, Kevin's right - Serrasalmus elongatus. Most definitely a solitary fish, he's an angry bastard. I've had him for almost 2 years now, very cool fish that you can actually keep in a normal sized tank for life.
Quick video of him swimming around the tank. Looks to be right around 9" now. Huge difference since picking him up at the end of last August - I just watched that first video of him swimming around the tank and compared it to how he looks in there now, surprising how much he's changed over a fairly short amount of time.