Well let's think logically.
The opposite of a fish would breath air.
The opposite of a fish would not have eyes
The opposite of a fish would not have scales or fins
The opposite of a fish would not eat bugs/other fish
The opposite of a fish would not swim or go in water
I'm going to say a worm, but there are problems with that - it does need moisture for it's skin and some people eat worms, as well as fish