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Fish Anaesthetic

I have seen people with extraordinary skills catch a fish and inject it swiftly. The fish is clamped between the upper arm, forearm and the body of the handler and injected. This quick injection technique is valuable for wholesalers and breeders who must treat a high number of fish and the possible resulting losses are an accepted risk. This technique is risky especially if the substance to inject needs a prolonged injection time like Nuflor. The sharp bevel of a 20 gauge needle can inflict serious internal damage to a fish if it struggles during the injection.


This kind of “grab and stab” approach is really not necessary for the Koi hobbyist where every Koi is a treasure or a show specimen. The skill to anaesthetise a Koi will be a necessity and may be required from time to time to save a koi’s life. This skill may be required to do topical treatment of ulcers, to inject a fish or maybe to remove an object stuck in the throat. Unfortunately it is also sometimes necessary to take it to the extreme and humanely dispose of a Koi.

When one anaesthetises a Koi, there are various stages the Koi goes through. Once placed inside the prepared solution, the following occurs:

1. Fast movement of gills sometimes frantic “spitting”
2. Deceleration of gill movements and partial loss of sensitivity to touch
3. Loss of equilibrium, slow movement of gills but fish will react when handled
4. Fish are lying at the bottom of the treatment tank, infrequent gill movement  and not responding to handling
5. No gill movement and limp. Critical stage and may be fatal if left in the mixture too long.

Upon completing the procedure required. The fish is gently placed in a well aerated recovery bowl, or supported in the pond until it can swim away. Some people will place a fish on a wet towel to inject, but I have found that injecting a Koi while it is still in the treatment bowl is much less traumatic for fish and owner.

Clove oil (Eugenol)

Clove oil is distilled from various parts of Eugenia caryophyllata., Its active ingredient, is eugenol. Clove oil is a good alternative as a fish anaesthetic, because it is relatively inexpensive, easily obtainable from pharmacies and is generally regarded as safe for the user and for the fish.


How to use clove oil:

Because there are different concentrations of clove oil on the market, I normally start by using 1 ml of clove oil per ten litres of water

Clove oil does not dissolve in water so some agitation of the water is necessary after putting the measured amount of clove oil in the water. Some sources promote the idea if mixing clove oil in alcohol before adding it to the treatment water but I have found it unnecessary in water temperatures above 16 degrees Celsius. This mixture should immobilize the fish within about 3 minutes.


MS-222 is also known as, Finquel, Tricaine, tricaine methanesulfonate and
Metacaine. The medical use of MS-222 is mostly a local anaesthetic used by doctors and dentists.

How to use MS-222:

The dose is related to size of the fish, as well as water temperature and hardness. Generally doses are between 25 to 100 mg/L. For excessively long exposures, dosages at 50 mg/L or more should be avoided as mortalities may occur. As mentioned, anaesthesia and recovery time depends on the size of the fish.

Aeration should always be used, and I normally ad one teaspoon of Bicarbonate of soda to the treatment bowl as a buffer.


Phenoxyethanol (Phenoxetol)

Phenoxyethanol, also known as phenyl cellosolve, phenoxethol or phenoxetol, has a broad spectrum activity including gram-negative and gram-positive bacteria, yeasts and moulds. It is a versatile preservative which has a wide range of uses from detergent based products such as foam baths through to skincare foundations and make up. This is my favourite anaesthetic. Phenoxetol is stable and effective over a wide pH and temperature range.

How to use Phenoxetol

The dosage for Phenoxetol is 1 ml per liter of water. Closely monitor the fish as it goes through the various stages. When the fish is sufficiently sedated, it can be injected and then released in the recovery bowl or pond.

Euthanizing a fish

Euthanizing a fish is a very unpopular but necessary topic, because there comes a time when the hobbyist needs to euthanize a fish, for various reasons.

There are some unacceptable methods of euthanasia employed by people that includes freezing, boiling, chopping, removing the fish from water, or flushing it down the toilet. These methods are slow, torturous, stressful, and violent.

The best and most humane way of euthanizing a fish is basically to overdose with the above anesthetics and leave the fish in this mixture for an extended time. There are risks involved, like a fish coming around after it was presumed dead. There is however steps to ensure that it do not happen. The first step is to make sure the fish has stopped all gill movement for at least an hour. Transfer it to a plastic bag and place it in the freezer. The second step if you have used eugenol is to ad 20%   alcohol to the treatment bowl when the fish is already sedated. In both instances, the fish simply goes to sleep and never wakes up.

Hopefully you will rarely have to perform this task, but when you do, it's at least comforting to know the fish did not have to suffer.

Last Updated on Wednesday, 29 February 2012 12:49