As fish keepers none of us want to think about loosing our favorite fish but unfortunately like all other living creatures they can become ill. When their illness cannot be cured with medication we sometimes come to the conclusion that the best course of action is Euthanasia.
The most humane Euthanasia method is the clove oil method which is the method we recommend. It is a two step process involving the use of clove oil and white grain alcohol (vodka). Clove oil, or eugenol, is available at drug and vitamin stores. Clove oil will put a fish to sleep and ensure it feels no pain. The fish can wake up from this sleep if removed from the clove bath, however. The last step of adding the vodka will ensure the fish expires.
For small fish up to 3" (7.6cm) tank water should be placed in a small container, just large enough for the fish, measure the amount of water used, you will need this later to determine how much white grain alcohol (vodka) to add. Next fill a small bottle with tank water, a small pill bottle works well, add one drop of clove oil and shake vigorously. The clove oil should emulsify, turning the water milky white. Pour 1/4 of this mixture in the container holding the fish. The fish will begin to fall asleep you should wait at least ten minutes. After ten minutes the fish should be resting on the bottom and will look like it has expired. If after the ten minutes the fish rises off the bottom and swimming intermittently, shake the clove oil mixture and add the same size dose again. Wait another ten minutes before you proceed to make sure the fish is asleep.
Once the fish is asleep add 20% to 25% white grain alcohol (vodka). For example, if the fish is in 8 oz (240 ml) of water, 2 oz (60 ml) of vodka should be added. After 20 minutes, the fish should be checked carefully for any gill movement. If there is no gill movement over a 60 second period, the fish has expired.
For larger fish the dose of clove oil is increased to 10 drops per gallon of water used. This should be mixed in a small bottle with tank water before use. For example if the fish is placed in 3 gallons of water 30 drops of clove oil should be emulsified in a small amount of tank water. Add this mixture slowly to the holding tank wait at least 10 minutes. Once the fish is asleep add 20% to 25% vodka wait at least 20 minutes and confirm that there is no movement. If there is no movement or gill activity for at least 60 seconds the fish has expired.
We all hope that you will never have to do this to any fish you keep but if it becomes necessary this is the most humane method.