Aquatic animals, like other pets sometimes need to be killed humanely to prevent them for suffering unnecessarily.
At all times pet euthanasia methods used must comply with local laws such as those regulating the use and disposal of toxic substances. The summary below is not an endorsement of the safety or legality of any method--it is an easy-reference summary of various other sources of information (listed at the end). You are encouraged to consult with your veterinarian to determine the most appropriate method to use.
Aquarium animals and plants should never be released into the natural environment. It is the responsibility of the owner to rehome or euthanise any animal they no longer wish to own.
With any animal a chemical that first sedates, and then kills, the fish is considered the most humane non-physical option. Commercial fish sedatives such a FinQuel (also called Tricaine or MS-222) will cause a humane death at high enough dose and exposure--double the dose required for sedation is commonly recommended. As the need to euthanise may arise unexpectedly, it would be wise to keep a bottle of FinQuel available. FinQuel should be stored in a tightly sealed container at room temperature--and never used on fish that will be eaten. Avoid inhaling Finquel or having it in contact with your eyes--use of a face mask and gloves is recommended.
To euthanise a small fish with Finquel place the fish in a 1 gallon container with water from the aquarium, or similar dechlorinated water. Add one half teapoon of Finquel powder and one half teaspoon of baking soda a small amount of water and add to the contain.
A viable alternative to FinQuel is the use of clove oil which is less expensive and requires fewer precautions. However clove oil is better introduced gradually, a few drops at a time.
Alternatively baking soda added to water increases carbon dioxide which causes narcosis. This will only be effective in water below PH 7, and so you may need to pre-treat the water with a pH decreaser. Add one tablespoon of baking soda per cup of water and fully dissolve. Add the fish to the solution using a net.
Regardless of the method, wait at least 15 minute and until the fish has ceased all movement including gill movement and is prone in its side. Due to the potential for recovery from any sedative, death should then be completed assured by a physical method or freezing before disposal.
Dombrowski (2007) recommends the following methods for veterinary euthanaisa of aquatic invertebrates such as snails:
* tricaine methanesulfonate (1-4 g/L)
* magnesium chloride (10%)
* dilute ethanol (10%)
followed by immersion in ethanol (70%) or freezing. But concentrate ethanol or freezing should not be used as the primary form of euthanasia for humane reasons.
Pharyngula: How to euthanize a fish
UltimateBettas: Euthanasia Techniques
Argent Labs (small order charge of $15)
Drs Foster & Smith