A number of quick, relatively painless methods have been devised to humanely dispatch fish. A commonly used method is clove oil to anesthetize the fish, followed by an alcohol bath to kill the fish. Another option is to add eight Alka-Seltzer tablets per gallon of water, which releases carbon dioxide to kill the fish quickly. However, RateMyFishTank.com suggests freezing the fish afterwards to ensure that it is dead.
To use clove oil to euthanize a fish, first move the fish to a small container of water. Put three drops of clove oil in 8 ounces of water and shake well. Pour the oil mixture into the fish's container. Add more clove oil if necessary. If sufficient clove oil is added to the fish's container, eventually the fish dies, says Oscarfishlover.com. However, because fish sometimes do not die from lethal clove oil doses, it may be necessary to use alcohol to kill the fish. Pouring alcohol (vodka is usually suggested) into the fish's container kills the sleeping fish, says RateMyFishTank.com.
Tricaine methanesulfonate is a commercial fish anesthetic. Applying a double dose humanely kills a goldfish, reports RateMyFishTank.com. However, this drug is not widely available for purchase.