Euthanizing anything via freezing is painful and distressing. We had a client that put down a sick bird in the freezer then brought it in for necropsy (pretty efficient way to ensure we wouldn't be pulling accurate swabs!) and the horribly contorted state the bird was in gave every indication that it was NOT quick and painless. The only time I have ever used cold to euthanize was when I got a baby turtle that was so horribly mangled that it was a crushed mess. Rather than try to pull its head out to get to the base of its neck and cause it more pain I submerged it in an ice bath and the sudden shock of temp changes literally killed him instantly. I learner that from a visiting wildlife biologist at one of our clinics. This little turtle probably weighed 70 grams, though. The short of it is that freezing fish is not a desirable means of euthanization. There is also the fact that they can't breathe adequately adding to the pain of cellular destruction.