I know some people say a few drops of clove oil before putting it in the freeze causes the fish to lose consciousnesses.
honestly the best way to kill something is fast, hard, and never miss. I have been in situations before that have been pretty bad and required immediate euthanasia, like when my dog escaped our house and was hit by a slow-moving truck just outside of our driveway, he was hit with just enough force to break 3 legs, several ribs, ruptured his intestines and puncture a lung. I had a relative go back in the house and bring me my gun and we ended it then and there. That dog was not going to make it and I wasn't going to make him suffer. It was messy but they only thing I could do.
I would do this with any pet that really needed it. The few times I have had to take out a fish I have just taken a heavy brick to them (for the small guys). For the bigger guys, I have used the disposal, and a blender on high (Seems bad, but they're gone in a second and don't feel a thing)
I see freezing reccomended for all kinds of animals, especially the ones deemed not "worth" taking to a vet (such as hamsters, rats, fish, etc), and as a result a LOT of small pets really suffer in the freezer. In small pets, the extremities can suffer severe frostbite before the host dies. They stop moving because it becomes painful to move muscles once they fall under a comfortable temperature.
If you have to take anything down, then it should either be at a vet, or, if you are SURE you can do it 100% properly NO chance of mistakes, at home, with a proper, quick, painless death, even if it makes you feel bad or uneasy. If you can't stand to watch it, you shouldn't try to kill it at home!