Gosh I am soooo sorry u are in this situation. I chose to euthanize a seriously injured fish once, after much studying of what would be the most humane way.
I would NOT recommend freezing, apparently having it's brain crystallize during the process is NOT painless, and I don't think I could ever chop off my fish's head
. Clove oil, which can be purchased in most health food stores, has been used as a fish anesthetic for many years. If gradually added it will put your fish to sleep, and eventually at the right concentration, end his suffering. I have heard the reco of adding a couple alka seltzer tabs, one at a time, AFTER he is deep asleep because the high C02 levels will bring the end about a bit faster, without increasing suffering.
Firstly, move the fish into a smaller tank,(if it is in a larger tank)- something that can be thrown away or easily cleaned. If you do this proceedure in a large tank i wouldn't know how much clove oil to use and also you will end up with a very potent smelling, greasy poisonous tank (the smell takes a while to go) Clove Oil, or "Oil of Cloves" is an essential oil used in foods to flavour and used as a "natural" anaesthetic.
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. Add another mixture of 2 to 3 drops of oil in water. When the fish goes "belly up" it is asleep - not dead. Then add 3 more drops of clove oil. The fish feels nothing, it is very peaceful and humane. It seems mad that a few drops of something can have such an effect, but it IS very strong. 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 proceedure 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.
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