I have, unfortunately had to put a very sick fish to it's death, so it would not suffer. It was beyond help.
I learned from a local fish store, that if you take the fish and put it into a small bowl or container, using the tank water that it's in. Then drop a couple alka seltzer tabs into the container, or zip lock back works ok too.
First hand experience, this ended the fishes suffering imediately. Very quick.
I don't like the idea of the sharp blow to it's head, because I fear that if I don't do it hard enough, it would cause the fish pain and further suffering. If I do it too hard, it would make a mess and make insides splatter. I don't want either to happen to a fish I love.
I don't like the freezing the fish in the freezer routine because many people do say that it is a slow painful death.
To me, the Alka Seltzer is the kindest, fastest and most humane dignified way to end a fishes suffering.