Question

which is the venomous fish

Answer 1

Venomous fish produce a strong toxin harmful to humans (called venom) which they deliver by means of a bite, sting, or stab. This results in an envenomation. As a contrast, poisonous fish also produce a strong toxin, but they do not bite, sting, or stab to deliver the toxin. Instead they are poisonous to eat because the human digestive system does not destroy the toxin they contain in their body.

Lionfish was though to be the most venomous fish, but the stonefish has now been indentified as the most venomous one. 
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Answer 2

There r many venomous fish like lionfish ,puffer fish ,electric eel , sting ray, and some types of fish