if you research this you will find that the frozen water method has been tried and tested by laboritories and animal welfare societies and is found to be 100% humane - FOR tropical fish only NOT cold water fishies
the method i am on about is not the one where you put the fish in the freezer but the one where you put a container of water in the freezer until there is a solid lump of ice on top , you then break that ice and place the fish in the water
Having done this i know that the fish stops moving immiediately after being put in the water - there was no gill movement at all
i left the fish there for 10 mins just in case but it never moved again