the most aggeresive fish

I would have to vote for the Oscar and the labidochromis caeruleus. My grandmother had 2 oscars. One poked the eye out of the other oscar. And when my daughter goes up to the tank one of the oscar attacks the glass where my daughter is. It's as if it wants to eat her face off. As for my labidochromis caeruleus, they've killed a betta I had, some guppies I had, they try and eat the dorsal fins off of my moonlight gourami, they are always fighting with my pleco, and they all alway fighting with each other. I also have some long tank tongs that I use when I need to put a plant or new decoration in. Well, when I put it in the tank the labidochromis start attacking it.

really shouldn't keep labs with gourami/bettas etc.

they're about as pleasant as africans get but far too much for those fish.

pound for pound I would say the nastiest is melanochromis auratus. true tank killers. followed closely by pseudotropheus demasoni.
 
spotted pike carason. it is illegal i hear, but my cousin had one and it was vicious. it killed anything in the tank, he even put in a mouse and piranah and it killed both.
 
Alligator Gar ... they get 4 feet and have huge teeth, they will eat anything they can fit in there mouth, which is almost everything freshwater. If were talking Salt water too then I would go with a Killer Whale
 
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