DEADLY fish
The stone fish
jelly fish
The tuna fish is not at all deadly to humans. It is a staple of many people's diet throughout the world.
stone fish
Humans, Snake, animals
Piranha.
Fugu, which Japanese for Puffer Fish, can be quite deadly if any of the tetrodotoxin in the organs contaminates the meat during cleaning of the fish and/or during preparation of the meal.
Blow Fish is another name for Puffer Fish. They are poisonous but I believe in some specialist Japanese restaurants the fish is prepared by experts and eaten by the brave.
the snake is only deadly to fish, but not to humans. it will take allot of bites to in pulse venom to interact to humans ( like one billion bites to be deadly) and is less than one percent deadly to humans.
Neither Reptiles nor Amphibians should be kept with fish. They will poison the water for the fish making it deadly.
Any sulphate (sulphur) at all would be considered dangerous and potentially deadly to fish.