My understanding is the freezing method is only humane with coldwater fishes, like godlfish, that automatically hibernate at a certain temperature. There is, apparantly, some debate as to whether fish feel pain at all. But I think it is better to side on the error of "they do" than they don't. I would use clove oil or possibly the most humane method [which I have never been able to do] is to quickly decapitate the fish. I would not do that unless you really know what you are doing however. You could make a mess of it and create suffering in the animal.