I took this from goldfishinfo.com in the human disposal method:
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Preferred Method: Clove Oil and Tank Water Method
Buy pure clove oil. You can get it at a health food store for under $10 for a ½ 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.
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Then here's something on the freezer method from the same site:
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I read your humane disposal of fish section. When cold blooded animals become chilled they cannot move so their death appears peaceful but there is some evidence (I cannot remember where I read this as it was 7 yrs ago when I was researching this subject in regards to humanely killing frogs). However, the animal can feel the pain of ice crystals forming in the body while it freezes. The eugenol method as written above is far more humane since I believe it is an overdose of anesthetic which stops nerve function thus eliminating pain. We switched from freezing to using an overdose of an anesthetic (the active ingredient in Anbesol) for frog euthanasia.
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So it looks like from these posts, the best way would seem to be the clove oil solution making sure it is mixed around well enough, and then freezing to make sure. Shoot, even after that, I'd also cremate JUST to make sure. I can somehow see the fish re-animate or something after the ice melts around him
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The freezer method actually causing pain makes sense to me, but I can't be sure. Overdose of the anesthetic and then the freezing seems the best way to me (I am NOT saying I'm 100% sure though).