Unfortunately it is sometimes necessary to kill fish humanely. It is now accepted that fish feel pain or something similar to it, therefore euthanasia must be performed using a technique that kills quickly and with as little pain as possible.

Therefore, it is generally considered that a chemical method is the most appropriate way to humanely kill ornamental fish, usually by administering an fatal overdose of an anaesthetic agent. This involves immersing the fish in an appropriate concentration of an anaesthetic agent which has been mixed into the water.

As a guide, a dose of 5-10 times that used for anaesthesia will produce rapid and effective death. It is recommended fish should remain immersed in the strong anaesthetic solution for a minimum of two hours to produce brain death. To ensure that death has taken place (as opposed to being comatose), it may be necessary to follow the anaesthetic overdose with a ‘killing’ procedure, such as severing the spine with a sharp knife just behind the head or decapitating the fish.

Dead fish should be disposed in a safe way, and in accordance with current disposal of waste legislation. The body must not be flushed down the toilet.

Anaesthetic agents and dose rates for euthanasia

In order to carry out euthanasia, we recommend using water taken from the aquarium/pond from which the fish has been removed. This ensures the water chemistry is the same and avoids further stress to the fish.

This water is then placed into a smaller, separate container and the anaesthetic agent is then added. Some agents such as Tricaine will cause the water to become more acidic therefore sodium bicarbonate or other buffering agents should be added to ensure the pH is increased to that of the original aquarium/pond. A pH meter is required for accurate measurement.

A number of anaesthetic agents are available and routinely used for the euthanasia of

ornamental fish although none are licensed for euthanasia. Tricaine Pharmaq® is licensed as a fish anaesthetic and is available only on prescription from a vet but there are also a number of anaesthetic agents available ‘over the counter’ that do not require a prescription.

Here’s a round-up of the anaesthetic agents suitable for the humane euthanasia of ornamental fish, with the recommended euthanasia dosage per litre of water:

Tricaine Pharmaq® (tricaine methane sulphonate, MS222®)

This is currently the only licensed fish anaesthetic agent and should be considered asthe first drug of choice. Tricaine will cause the water to become more acidic and a pH buffering agent should be added to the solution. It is prescription-only so is not available over the counter.

Euthanasia dose: 1 gram/litre.

Available ‘over the counter’

Benzocaine

This chemical is relatively insoluble in water and should be made up as a stock solution with 100 grams of benzocaine in 1 litre of acetone or ethanol.

Euthanasia dose: 10 mls of stock solution/litre.

2-Phenoxyethanol

This chemical does not dissolve well in water and must be whisked vigorously to improve its solubility. In the UK, it is available without prescription and sold over the counter as Aqua-Sed® (Vetark Professional).

Euthanasia dose: 2.5 ml/litre. If Aqua-Sed® is used, the euthanasia dose is according to the manufacturer’s instructions.

Clove oil

Sold as Koi Calm® (NT Labs) and also available in pharmacies without prescription, this drug must be whisked vigorously to improve its solubility. It is suggested that the euthanasia dose should be added slowly over a five minute period so as to avoid distressing fish.

Euthanasia dose: 10 drops/litre.

Unacceptable methods of euthanasia

Do not attempt to kill your fish using carbon dioxide, removal of the fish from the water (i.e. asphyxia in air), maceration, freezing or flushing down the toilet.