Humanely Euthanizing Fish
Clove Oil and Tank Water Method
Buy pure clove oil. You can get it at a health food store for under $10 for a ? ounce bottle. Put the fish in a medium sized mixing bowl in his own water from his tank. In a small jar or something with a lid (I use a cleaned out jelly jar) mix the clove oil with tank water. Put the lid on and shake it like crazy over and over until the liquid in it is white. Then pour a little into the mixing bowl with the fish. Swirl it with your hand. The fish might fight it just a little bit and then slow down. Then pour a little more in and swirl again. He should just go to sleep and appear dead. If he doesn't, try a little more of the clove solution, always shaking very well before an addition to the bowl. When he goes to sleep, leave him in the solution for a good 10 minutes and then put him in a small cup or zip-lock baggie and put him in the freezer. Pain free death. Very humane. We should all go so easily.
Clove Oil and Vodka DON'T USE!!!!
The clove oil and vodka method is often sited as a humane method of fish euthanasia. Clove oil (eugenol) is used as an anesthetic in fish for surgery. The vodka is used to disperse the clove oil in the water. The clove oil/vodka mixture is placed in water and then the fish is added and supposedly dies peacefully. This doesn't work! Use the clove oil and tank water method described above.
I believed, after hearing about a Betta struggling and thrashing when placed in the clove oil/vodka bath (IN WATER), that there must be a better way.
Tricaine Methanesulfonate (TMS)
Another method I was told about is another fish anesthetic called Tricaine Methanesulfonate (TMS). One gram of TMS in one liter of water makes a bath that will put the fish to death peacefully. I haven?t tried this, so I can?t personally recommend it.
An old and still widely used method of fish euthanasia is to place the fish in a container of its own tank water and place it in the freezer. I have done this, but have never been completely comfortable with it.
Using a knife
Slice the fish right behind the gills. Cut all the way through and take the fish?s head off. Quick and painless, if you can do it.
Net the fish and slam them hard against the floor, a wall or a counter-top. An alternative, and not quite as traumatic to the fish owner would be to place the fish in a bag before slamming. I don?t believe this would work very well with very small fish as their lack of weight may make this method ineffective.
In the end, I believe the most humane way to euthanize a fish is with the clove oil and tank water method. I have used it myself and can verify that it is quick, humane and apparently painless.