From Riso
I believe that the reason people often use the method is because many fish are already used to being cold. So, if it just gets colder than before, the fish either goes numb, or goes unconscious. There's a sticty at the top of this forum discrbing humane ways of doing this. I heard clove oil is also used, you may want to ask other members about that, it's suppose to cause them to peacfully fall asleep and of course they don't wake. There are also a few articles on this site about thi subject, and also many on other sites as well.
By the way, he method you are using is a bit traumatic, but if it works quickly, then I leave the choice of method to you. Good luck.