Clove oil is a sedative. You catch the fish into a container with some hiding places, of maybe a gallon of water. Then you take a bottle half-full of tank water, add a few drops of clove oil to it, and violently shake the bottle. This mixes the clove oil into the water. Slowly add the mixture in the bottle to the container the fish is in, with additions 5 or so minutes apart. There are probably numbers for this online, but I've done this a few times without any actual measurements. It's not super precise when you're trying to kill the fish as opposed to sedating it alive.
You'll see the fish slow down, and eventually it'll act like it's asleep. Keep adding clove oil. Once it's at a point where you can poke it and it doesn't react at all, put more clove oil into the bottle, shake again, and add a bunch to the fish container. You want to add the clove oil slowly so the fish won't be alarmed by the smell, but once it's unconscious, you don't have to worry about scaring it.
After you've done that, leave the fish in the container for a few hours, potentially overnight. Eventually it'll stop breathing. You leave it in the container for a long time to make sure it's actually dead and won't revive, as fish can still absorb some amount of oxygen without their gills acting, and as such can be revived if they're removed from the oil mixture shortly after they stop breathing. It's only as stressful as the process to put the fish in the container, and causes no pain. It's a good, gentle way to euthanize a fish. Plus, clove oil is cheap, relatively easy to get, and completely safe to handle. Just wash it off if it gets on your skin, essential oils are highly concentrated and shouldn't be left on the skin without heavy dilution.
