I personally think that you did the most humane method available to us.....rapid cold immersion. To help you feel better about this, here are some thoughts for you.
Fish volume is so small that rapid immersion will almost instantaneously drop a tropical fish's temp down to that of the water. Cold temps deaden nerve transmission, so, cold is a topical anesthetic. Not only that, the brain will rapidly become unconscious, so you have decreased pain and unconsciousness, two of the main ingredients for a humane euthanasia.
If what humans experience can be roughly translated to fish (no one will ever know for sure), hypothermia is supposed to be a euphoric experience. I think you did the right thing.
Sorry for your loss. SH