Alka

Alka Seltzer is a popular source for carbon dioxide to be released in water.

One method that has been recommended by a non-veterinary (but experienced Oscar breeder) type is the use of Alka-Seltzer. Place the fish in a shallow container of water and place 2 Alka-Seltzer tablets in a position under the gills.

1) Anesthesia : Add 1 tablet of Alka-Seltzer / 20 liters.
2) Euthanasia : Add 30g of NaHCO3 and dissolve completely. Add the fish to euthanize - wait 10 minutes after all breathing stops.
Tricaine (Finquel) ...

This can be put into the water via airstone, Sodium bicarbonate (Bicarbonate of soda, baking soda), health salts (Alka-Seltzer). It is not recommended for amphibians.
Pros:
Easy, and painless.

There are several acceptable ways to kill a fish: severing the spinal cord just below the head with a sharp pair of scissors; placing the fish in water and rapidly cooling the container with ice; or overdoing with an anesthetic (Alka-Seltzer®).

Drugs and Alka-Seltzer didn't work. In frustration, I tried beating them against the wall but the neighbors complained.

See also: Water, Stress, Aquarium, Fish, Temperature