Actually, depending on the fish, flushing it in the toilet likely COULD release it into the wild. The toilet just goes to the sewer, which while nasty, is a place fish could survive, especially something like that. All it takes is one heavy rainfall to flood the sewers, and tada! You've released a fish into the wild.
If you're going to dispose of a fish, do it humanely and then bag it and trash it, or bury it somewhere. If its alive when it leaves your hands, there's no telling where it will end up.