I use blunt force trauma when I euthanize fish. I wrap them up in a damp cloth to lessen the stress, and then after locating the head, use a hammer to deliver a fatal blow. None of my fish have even so much as twitched afterward, and it happens so fast, their brain doesn't have time to process it.
I'm sorry it came to this for you. It made me feel horrible the first time I had to euthanize one of my fish, but if there's zero chance of recovery and a slow, lingering death ahead of them, it's our responsibility to end their suffering.