What is the most humane way to euthanase aquarium fish?

The term euthanasia comes from the Greek language meaning a good death (eu = good, thanatos = death). Euthanasia of animals to end suffering and pain is widely accepted as an important component of safeguarding their welfare, providing that the euthanasia process minimises or eliminates pain, anxiety, and distress prior to loss of consciousness.

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 their adaptation to different environments (e.g. temperate versus tropical, fresh versus salt water) [1].

The main reason for considering the euthanasia of a fish will be that the fish appears to the owner to be suffering or terminally ill. There is anecdotal evidence suggesting that when an owner thinks that a fish was sick or dying, they may decide to dispose of the fish. The methods of disposal used include flushing the fish down the toilet or using some other crude method. As discussed below, many of these are not acceptable methods.

Anaesthetic overdose

An overdose of an anaesthetic agent dissolved in water is the most humane method available for the euthanasia of sick or injured aquarium fish.

Clove Oil (that contains eugenol) is a sedative which, at high doses, can be used to euthanase fish. Clove oil is readily available from most chemists. To assist with accurate dosing, it is recommended that, whenever possible, products with standardised, known concentrations of essential oils be used. Approximately 0.4ml of clove oil per litre of aquarium water is sufficient to cause death in exposed fish. For this method of euthanasia, some of the aquarium water is placed into a clean bucket, and then the fish to be euthanased is caught and placed into this bucket. The clove oil should be mixed with a little warm water first before it is slowly added to the bucket containing the fish to be euthanased. The clove oil must not be added all at once as this can cause the fish to get excited – rather the clove oil mix must be added slowly over a 5-minute period. Clove oil appears to be less aversive to fish than MS-222 (another commonly used fish anaesthetic) suggesting it is a more humane euthanasia method [2, 3]​​.

When exposed to clove oil at this concentration, fish quickly lose consciousness, stop breathing and die. Please note that the fish must remain in the solution for at least 30 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 ensure the fish is dead and can then be disposed of safely.

It is strongly advised that advice is sought from a veterinarian with specific aquatic experience before euthanasing aquarium fish.

The other alternative is to take your fish to a veterinarian who can use a variety of anaesthetics to euthanase your fish. Further information on the use of anaesthetics to euthanise aquarium fish is provided in the American Veterinary Medical Association (AVMA) Guidelines for the euthanasia of animals (2020) [4].

Physical methods

Physical euthanasia (stunning, pithing, decapitation) methods require competent fish handling and can cause considerable stress to 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.

Inhumane methods

Some methods of fish euthanasia are inhumane and must not be used. These include:

• flushing live fish down the toilet
• placing the fish in the freezer
• boiling
• decapitation without stunning or pithing
• suffocation by leaving fish out of water

Verifying death

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 of a corneal eye reflex (touching the surface of the eye).

Disposal of the body

Correct disposal of the fish body is important to minimise any possible risk of disease spreading to fish populations in local rivers and streams and to respect the body of the fish. It is not acceptable to flush fish bodies down the toilet, dispose of the body in natural waterways, or simply throwing the body in the garden.

There are two alternative methods. The first is to wrap the body in a paper bag and bury it in the garden. This would need to be deep enough that foraging birds or animals could not find and eat the body, with greater than 30 centimetres depth being sufficient. It is important that the burial site is more than 100 metres away from waterways. The second is to wrap the body in a plastic (compostable) bag and place this into rubbish that will end up in a landfill situation, such as normal commercial or residential rubbish collection. If it is going to be some time after the death to the rubbish collection, it will be advisable to freeze the body until the time of the rubbish collection.

For more information, we recommend that you talk to an experienced aquarist (fish carer) or fish veterinarian.