Hi Stew,
There are a number of ways to euthanize. I have done the fish in the freezer method...as the temp of the water drops the fish will slow it's respiration and stop. But I also heard that this is not the most humane way. Now I use clove oil...net the fish out and place in a small bucket then add clove oil. The clove oil is an anesthetic and at a high enough dosage, the fish will simply stop breathing and pass away.
HTH, Lester