Definitely Pygopristis denticulatus. It's a piranha that prefers seeds and fruits, but will also eat fish (I had a shoal destroy a Channa bleheri). It's actually quite cool to watch them go to town on a handfull of birdseed.......... they just don't eat the seeds, they actually husk them with their cusped teeth and spit out the shells! Very cool fish, definitely worth keep, and one that really prefers company of their own kind. I miss my shoal of 10.
Sorry, but I can't find the pics of the ones I used to have, but I can tell you with 100% confidence that the fish you have is Pygopristis denticulatus.