5. Mosquitofish, Gambusia affinis sp.
I used to think these things were just guppies and were boring. Then I saw how a school turned a 2 1/2ft Koi into a dead fin-less pulp. Oh sh*t. I now have a school in a 40 w/ a pair of Cons (the only thing that can really stand up to well-established school of these little Pirahna's). When the food hits the water, they go nuts. They hit harder than Bass (scale wise). A lot of the time, they actually come out of the water to hit a pellet. And they will shred moths and stuff I drop in there too. Once I put a feeder goldfish in there to save it for later as everyone was full (before I stopped feeding live) and instantly they pounced on it. They completely took the tail fin off and the pectorals looked like shredded flags. They also took both its eyes out and a few min. later, it died. They also, very quickly learned how to eat from my hand, which is sick. If these little things were Oscar sized, I would be scared to even go near the tank. IMO, they are the most viscious fish I have.