From the info you just provided, I would say Serrasalmus Maculatus, Gold Piranha. You could put (1) in your 50g. They can get pretty nasty, and even sometimes follow you around when close to the aquarium. They will eat live food, but based on my own personal experience with piranhas, feeding them live food constantly is only asking for problems. I used to give mine a varied diet of non-oil based food fish like tilapia & bass as well as frozen whole krill, carnivore pellets, & market shrimp, with live feeders only being a small portion of their diet; about once or twice a month. I only ever used guppies because there is controversy surrounding the growth inhibiting qualities of goldfish amongst other cyprinids; such as minnows, aside from that it is a proven fact that feeders are not as nutritious for the fish as a pellet based diet is. Lastly if $50 is all you are willing to spend, then you might have to start off with a smaller juvenile specimen and grow it out, the adult specimens usually sell for more.