How To Kill A Sick Pet Fish
Fish keeping has a dark side, and that is the end of life issues associated with fish. Sometimes it becomes necessary to euthanize a pet fish because of illness. The popular solution associated with this issue is flushing your fish down the toilet, however this not only doesn't kill the fish usually, it can endanger native wildlife if your fish manages to make it through the system and back into rivers. (It is apparently not uncommon for workers at water treatment plants to find live fish that have been flushed.)
So what can you do without traumatizing yourself and the fish unduly? And what doesn't work?
The Circle of Life
If the fish is not diseased, feeding it to a larger fish is always an option, and probably one of the more 'humane' ones, assuming that the fish is about the right size to fit in the larger fish's mouth. Large cichlids are usually predatory and will put disabled fish out of their misery very quickly.
If you have a blender, you could theoretically toss your fish into it whilst it is running. However this turns your fish keeping experience into something like a slasher film. It's bad enough having to put a pet out of its misery without also seeing it transformed into fish gut soup before your very eyes.
Squash, Hack and Slash
Obviously hitting your fish with a brick, or stabbing it in the head with a knife is going to kill it. Also rather obviously, this is usually not a preferred option for most owners. If you really have no other option and it is a small fish, you can put the fish in a plastic bag, cover the plastic bag with paper and hit it with a brick. This makes for a quick death and minimal clean up.
In my experience this is the fastest, most pain free and humane way of putting a fish to sleep. Clove oil is a natural anesthetic, and at low concentration levels can be used simply to put fish to sleep for a little while. At higher doses, and without removing the fish from the solution, the fish will die.
The dose rate for death is: 0.25 ml per liter of water (about 1 ml per gallon, or 1/4 teaspoon per gallon).
If you intend on keeping fish, I strongly, strongly recommend you invest in some clove oil. There are some fish ailments that strike fish that are incurable (like dropsy), but which do not kill the fish right away. The fish can spend days or weeks slowly dying, and potentially infecting other fish.
Using clove oil at the rates mentioned above and leaving the fish in the solution overnight is a kind way to euthanize a water pet.