Do not flush a fish. It is not a quick, painless death as many people think. It is really not humane at all. Fish breathe through their gills, and when you drop them in the toilet and they travel down to the sewer, they are basically suffocating slowing on bleach and sewage. Not a nice way to die.

Put your fish and some of his own aquarium water in a ziplock baggie, and put it in the freezer. I put the baggie inside a brown paper lunch bag - that way I don't have to see the fish later. This is a more humane way to kill a fish. Since fish are cold-blooded animals, he will just get cold and sluggish, and go into a coma of sorts. He will be unconscious and then die, in his own treated water. Much more humane than suffocating on sewage.