If you go that route, clove oil does work, but MS-222 is better. IME, you need to be slow with clove oil, starting out well mixed, and then adding a bit more as you go (we're talking drops here). Here's the clove oil euthanasia dosage/procedure (courtesy of "the SH site"):
CLOVE OIL Euthanasia Dosage and Preparation Instructions
• Put 3 drops with a half pint of water and shake very well, so the oil and water make a fusion - otherwise the oil will just float on top of the water and for the euthanasia to work the fish has to get the oil into its system.
• Add the mixture to the water that the fish is in (1 gallon of water should be more than enough) and stir it around slowly with your hand. The fish should become lethargic and sleepy. When the fish goes "belly up" it is asleep - not dead.
• Then add 3 more drops of clove oil. Add another mixture of 2 to 3 drops of oil in water.
• The fish feels nothing, it is very peaceful and humane.
• Don't make the mistake of thinking that if you initially put more in it will act quicker - it will only freak the fish out - it has to be done gradually so the fish doesn't notice the change in the water.
• The whole procedure should take anywhere from 30 minutes to an hour. You will know when the fish is completely dead because there will be absolutely no movement of the gills. If after an hour the fishes gills are still moving you could add a couple more drops of clove oil. Sometimes it does depend on the size of the fish to how many drops of clove oil are needed.
• Properly dispose of the deceased fish and wash treatment container thoroughly with soap and water.
