From the article on this site I thought that:
"Temperature Shock, Freezing
100% humane, and the best way to go when euthanizing tropical fish.
The method is simple. Freeze a bowl of water until it has ice chunks in it, but is not solid in any way. Net the fish out of your tank and into the bowl. The difference in water temperature will cause an instant and fatal shock to the internal organs and nervous system of the fish. Sound harsh, but the deed is done painlessly and instantly. Make sure that the water is frozen sufficiently for this to be humane. Being impatient and dropping the fish in very cold, but not half frozen water, will only torture it.
Whenever someone asks my opinion on euthanasia, this is the method I suggest. It is the easiest to perform for the average hobbyist, and is instant. "
Was an ok method and this is what I have done when I needed to do it. Every fish I exposed to this seemed to expire instantly. Am I wrong?