By Patchin Curtis (extracted from a PVAS forum thread)
After enjoying Sherry's delightful presentation on ways to kill goldfish (very informative, thank you Sherry!), I did some research on Finquel and learned the following. Cornell University's procedure for euthanizing laboratory fish is to use tricaine methane sulfonate (MS-222), decapitation, or rapid freezing of anesthetized animals. MS-222 is acidic in solution and must be buffered by adding an equal weight of sodium bicarbonate or titrating to pH=7.0-7.5. Place the animal in a buffered solution of MS-222, at a concentration of >250 mg/l for fish or 2-3 g/l for amphibians. Immerse until death is achieved. Time to effect is proportional to MS-222 solution concentration. Chemical methods of euthanasia may require up to 3 hours before death results. A physical method, such as decapitation or rapid freezing, can be applied once unconsciousness is achieved. Finquel was made by Argent Chemical Laboratories which went out of business. In 2013 after the death of its founder. Tricaine-S (MS 222) by Western Chemical Inc. is available online in small quantities for the home aquarist.
Dosing levels for euthanizing fish: Fill a 4-cup measuring cup with tank water, add 1/8 tsp. Tricaine S, add 1/8 tsp. baking soda (to neutralize acidity of MS-222), stir and add fish. They very quickly settle and are dead within minutes or up to 3 hours. If you'd like to kill them faster, you could decapitate them once they are heavily sedated. I find this to be much more humane than grabbing them out of a tank and decapitating them.
In the past I have used clove oil, starting with a few drops in a small container with the fish and adding more as necessary until the fish is sedated and eventually dies. From my experience, a number of species of fish find the clove oil extremely caustic and show considerable distress. Some species take many hours to days to die, and anabantoids appear to be able to survive nearly indefinitely in a clove oil solution, even after adding dozens of drops to a small container, and in distress. After trying to kill a sick paradise fish I decided to never again use clove oil. That's when I switched to Tricaine-S.