Another contribution from my anonymous koi friend describing the use of clove oil. This procedure will generally hold for most anaesthetics, only the dose and time factors will differ.
Clove oil is a natural product ideally suited for anaesthesia of fish. The active ingredient is Eugenol.
Procedure for anaesthesia:
1. Add pond water to a bucket big enough to cover the gills of your koi and aerate the water.
2. Calculate the amount of water in the bucket.
3. Add 2 drops of clove oil per 1 liter of water in the bucket. (0.5 ml / liter) Because oil and water does not mix well, I first mix it with some of the bucket water by putting it in a small container which I then shake well before adding it to the bucket. An empty cool drink bottle works well. This also prevents you from accidentally releasing to much oil into the bucket.
4. Add the fish to the container and cover it for with a wet towel to prevent it from jumping out.
5. The fish should start to roll over a bit after 3 - 7 minutes. If not, add a bit more clove oil. The bigger the fish, the more required, and different batches of clove oil have different strengths. Gill movement will become slower, but should not stop. When adequately sedated, you should be able to lift it with support in the center and the tail should not flex much.
6. Now you can take the fish out of the bucket and place it onto the wet towel for treatment. Ensure that there is a soft surface. A bubble rap sheet works quite well. Hold the fish down with the same or another wet towel as it might jump.
7. Now you can treat. Work as fast as possible and get the fish back into the pond (supported) or another aerated container with clean water as soon as possible. It should recover within a few minutes.
8. If you only need to do a scraping, a half dosage can be used, although you don't even have to sedate the koi if you have experience in doing a scraping.
To euthanize koi:
Add 20-25 drops per liter of Clove oil to the bucket. It will send the fish into a peaceful deep sleep, from which it will not regain consciousness and will pass away painlessly and with the least stress.
For the more technical minded: Eugenol (C10H12O2) is an allyl chain-substituted guaiacol, a member of the allylbenzene class of chemical compounds. It is a colorless to pale yellow, aromatic oily liquid extracted from certain essential oils especially from clove oil, nutmeg, cinnamon and other herbs. Wikipedia.