I just decided to euthanize a pygmy cory that was very ill and covered in fungus/bacteria (the other cories in the tank are fine).
It looked bad, and b/c pygmy cories are both tiny and scaleless, I didn't think chemotherapy would be likely to save it.
I used the whiskey method. It stopped moving pretty quick, but it got pretty erratic for a few seconds when the booze hit the water (in a separate container).
After a few seconds, it was floating, but still moved when prodded and righted itself. I took it out of the container, put in another one with aquarium water, and put it in the freezer.
I don't know of what to make of the alcohol method. It may be fine...it's hard to fathom the piscine mind, after all, but I think I'm sticking to the freezer. Hopefully, I will not have to euthanize anything again.
Even though it is sometimes the most humane and responsible thing to do, it's still a bit painful and guilt inducing to have to put down an animal.