Thanks for pointing that out DeboraBremner.
I don't believe putting your fish down the garbage disposal is in anyway humane, and should only be used to dispose of the body. If you were to put a live fish down the garbage disposal the poor fish would die a slow and painful death just like flushing a fish down the toilet. (I believe, I am not certain) The most humane way would probably be clove oil but if you do not have any on hand, the best way and simplest would probably be the following which I found on another discussion page; the answer is by Lowryder:
03-13-2010, 9:56 PM
Temperature Shock, Freezing
100% humane, and the best way to go when euthanizing tropical fish. The method is simple. Freeze a bowl of water until it has ice chunks in it, but is not solid in any way. Net the fish out of your tank and into the bowl. The difference in water temperature will cause an instant and fatal shock to the internal organs and nervous system of the fish. Sound harsh, but the deed is done painlessly and instantly. Make sure that the water is frozen sufficiently for this to be humane. Being impatient and dropping the fish in very cold, but not half frozen water, will only torture it. Whenever someone asks my opinion on euthanasia, this is the method I suggest. It is the easiest to perform for the average hobbyist, and is instant.
Temperature Shock, Boiling
Again, 100% humane, the same method as above in regards to freezing shock, except you use boiling water. This method is recommended for cold water fish, instead of tropical fish, for obvious reasons. The temperature shock instantly shuts down all organs and the nervous system of the fish, painlessly. We do the same thing to lobsters when they are cooked. As always, use caution when working with boiling liquids. They wonâ??t kill you, but will sure leave some nasty burns. The same disclaimer as above. Make sure the water is at a good boil before employing. Dropping the fish in simply hot water, will not suffice.
Hope this helps!