How to euthanise a goldfish

Bubble became grew thin and then was found at the bottom of the tank.   We decided that we had to be cruel to be kind, but how?

After some research, spurred by my reticence at flushing a live goldfish down the loo, I discovered a comparatively humane way of sending Bubble to the ornamental pond in the sky. 

Apparently goldfish hibernate at low temperatures, so popping a goldfish in a plastic bag of its tank/pond’s water and popping that bag in the freezer is the relatively peaceful demise.  Much, much better than the single hammer blow someone suggested!