From: http://www.bettasrus.com/disease/euthanasia.htm

One formula that is very well known is Clove Oil & Vodka. The only
reason for the vodka is to help the clove Oil dissolve into the water.
Do NOT increase the Vodka in hopes it will help your fish die faster,
it will cause more discomfort. Increase the Clove Oil, if anything.
Clove Oil is used by veterinarians as an anesthetic to put fish to
sleep. Use this and your fish will do just that, fall asleep. Here is
the recipe:

First, mix 2 ml clove oil (can be obtained from a pharmacy or health
food store) with 8 mls vodka to make a 10 ml stock solution. Place
the fish in a container with one gallon of water and add the 10 mls of
clove oil and vodka to the one gallon of water, and the fish should
just go to sleep quietly with no struggle. Clove oil (eugenol) is used
as an anesthetic in fish for surgery, and the vodka is necessary so
that the clove oil will dissolve in the water. If your fish
struggles, it is most likely because the vodka level was too high.
Never raise the vodka level, please. The vodka may come in handy
afterwards to wash down the guilt ((

You could also use a fish anesthetic called Finquel MS-222. An
overdose will peacefully euthanize a fish. You can purchase a small
bottle of the stuff for under $10 from Argent Chemicals. Their website
is http://argent-labs.com/