The most agressive freshwater catfish in my opinion is the Whale Catfish, they may only grow to 10inches long, but that doesn't stop adults killing and eating any other fish it can bite into, alive or dead, regardless of the other fishes size.
Whale catfish;
http://planetcatfish.com/catelog/species.php?species_id=339
A couple of years back i saw this program on TV on this seriously deep river in Brazil, it showed the divers slowly swimming down to the depths of this river...On the bottom of the river they found vast numbers of whale catfish swimming around, so the divers decided to feed the catfish a large dead catfish they had caught earlier to film the feeding habits of these fish. However the divers didn't bargain on almost every whale catfish coming from miles around to feast on the corpse, and the catfishes ferocious feeding frenzy soon got so out of hand that the divers started to fear for their lives, the catfish decimated the huge catfish carcass so quickly that they started to pay interests in the divers, taking experimental nips on them.
The divers quickly surfaced as fast as they could and clambered aboard the boat, they looked so shaken up and sh*t scared they swore they'd never do that again.
So yeah, my praise goes to the adult Whale Catfish for being the most agressive and nastiest little freshwater beastie around, for its courage and downright determination to try and eat anything and everything thats made out of flesh and bones, alive or dead.
