There is no one method that can be considered as suitable for the euthanasia of all types of aquarium fish as they vary greatly in size and they are adapted to different environments. Also, the research that assesses the humaneness of different methods of killing fish is very limited, so it is hard to know exactly how humane some methods are.
The safest and most humane way we know for aquarium fish to be euthanized is by administering an overdose of anaesthetic dissolved in water. This method is especially appropriate for large fish that are difficult to handle but needs to be carried out by a veterinarian as the recommended anaesthetics are not available to the general public.
Clove oil is a sedative which at high doses, can be used to euthanase small fish. Unlike veterinary anaesthetics, clove oil is readily available. Around 0.4ml of clove oil per litre of water is sufficient to cause death in exposed fish. The clove oil should be mixed with a little warm water first before slowly adding it to the aquarium water containing the fish. Do not add all at once as the fish get excited – add the clove oil mix over a 5 minute period.
When exposed to clove oil at this concentration fish quickly lose consciousness, stop breathing and die from hypoxia. Note that the concentration of the solution must be appropriate for the size of the fish and the fish must remain in the solution for at least 10 minutes after all sign of gill movement has stopped. Once all sign of gill movement has stopped, placing the fish in the freezer until fully frozen will make double sure the fish is dead and can be disposed of safely.
Physical euthanasia methods require competent fish handling and can cause considerable stress to the fish. Many people are rightly uncomfortable with using physical methods. It is not recommended that you attempt physical methods without prior training as hesitation or lack of skill and experience will lead to a very poor outcome for the fish.
Whatever method is used, it is essential to ensure that the fish is dead before disposal. A fish can be considered to be dead 30 minutes after the last sign of gill movement and loss movement of the eye when the fish is rocked from side to side.
There are many methods of fish euthanasia that are not considered to be humane and must not be used. These include flushing live fish down the toilet; immersion in ice or placing in the freezer, boiling, decapitation without stunning first, or suffocation by leaving the fish out of water.
Please follow proper guidelines when making this tough decision.