A brown puffer or a leaf fish. Currently my meanest is an african butterfly fish or craayfish.
I don't possibly see how a leaf fish could be "mean" aka aggressive. That's my definition of "mean" anyway, not one that gobbles up any tiny fish. Leaf fish are designed the way they are for a reason: evasion, deceit, and to hide. They don't go about picking fights with fish lol. If by "mean" you mean ate a load of small fish, I have to protest, as that's simply in their nature...
We have to make sure by "mean" we're talking about actual fish that are pricks, not just predators acting normally.
Sorry for the rant, nothing personal, I just love leaf fish
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