why use anaesthetics for treating fish disease?
Written by Frank Prince-Iles

There are occasions when it becomes necessary to anaesthetize a fish so that it can be either examined or handled safely. A common situation is when there is the suspicion of a health problem and it becomes necessary to carry out a thorough examination.

While it is possible to check the general condition of a fish in a bowl or plastic bag, it is usually necessary to anaesthetize the fish to thoroughly examine any skin, gill or fin problems.

Anaesthesia is also usually required to carry out procedures such as a gill biopsy or scrape, injections and topical dressing of lesions. It really isn't possible to properly clean and dress a lesion on a fish that is flapping around. Although it is possible to carry out a skin scrape on an conscious fish, it is often better to lightly anaesthetize larger fish such as koi so that the scrape can be carried out properly and safely.

It can be dangerous

Anaesthesia and indeed many of the examination and treatment procedures described in these pages go beyond what would be considered as "normal" fish keeping practices. When it comes to anaesthetics it is important that we have a thorough understanding of their use, basic fish physiology and the potential problems. It is important to stress that all anaesthetics carry a real degree of risk and if we have any doubts about our ability to carry out this procedure responsibly and safely, then it is best to find someone to assist.

As with other techniques, if you do intend to try and learn fish anaesthesia it is best to try it a few times on healthy fish when all you have to worry about is "putting them under" and then bringing them around again, rather than waiting until you need to do it for real. This way you can become confident in using anaesthetics.

Planes or stages of anaesthesia

There are several stages of anaesthesia varying from light sedation during which the fish appears normal but there is a slight loss of reactivity - up to surgical anaesthesia during which there is a total loss of reactivity and the heart rate and respiratory rate slows. The final stage is medullary collapse, during which there is a total loss of gill movement followed by cardiac arrest!!!

The plan of anaesthesia reached depends on several factors such as dosage rate, period of immersion in anaesthetic solution, temperature, fish species and the condition of the fish. For most practical procedures we are looking to achieve a deep narcosis during which the fish has total loss of equilibrium, loss of muscle tone (keeps still), but can be easily revived.

Types of anaesthetics

There are many agents used to anaesthetize fish. Obtaining supplies of anaesthetics will depend on location - but all veterinarians should be able to obtain MS222 for you. Some of these anaesthesia agents can be obtained from the local pharmacy - or mail order from hobbyist magazines.

MS222 (tricaine methane sulphonate) is the only product licensed for fish use. It is a white water-soluble powder which is stable when kept cool and dry. It is a derivative of benzocaine (see below) A standard stock solution is made by adding 10g of tricaine to a litre of water - which should be stored in a dark container as it is unstable in sunlight. MS222 solutions are acidic and therefore the pH of the anaesthesia solution needs to be checked prior to use. In poorly buffered water it may be necessary to buffer the standard stock solution using sodium bicarbonate - maintaining the pH between 7 - 7.5.

MS222 is an hypoxic agent- so therefore the anaesthesia vessel should be vigorously aerated in use. Tricaine is used at a rate between 50 - 100mg / litre, which means adding 5-10 mls of standard stock solution to each litre of water used. Fish would normally recover in ten minutes or less.

Benzocaine (ethyl aminobenzoate). A colourless crystal or white powder that is a popular anaesthetic for fish. It is cheaper than MS222. Benzocaine is poorly water-soluble and needs to be prepared in either ethanol or acetone.

It has a reasonably wide margin of safety and doesn't require buffering. Benzocaine is more toxic at warmer temperatures. The stock solution should be kept in a dark container, and like tricaine it acts as a hypoxic agent, so aeration is required when it is used.

A standard stock solution can be prepared by adding 100g of benzocaine to one litre of acetone/ ethanol. This is then used at a rate of 1ml per litre of water. At these levels the fish should lose equilibrium in in 2-4 minutes. Fish should recover in ten minutes or less.

Eugenol or clove oil is often available over the counter at many pharmacies. It is not completely water soluble and should be mixed with ethanol or acetone at a standard rate of 1:10 (i.e. 100ml eugenol to one litre of solvent) This gives a working solution of 100mg of eugenol per ml of stock solution.

It is used at a rate of 40 -100mg per litre which equals 0.4 - 1.0 ml of stock solution per litre of water.

Other anesthesia agents include: Quinaldine Sulphate - light yellow crystalline powder, more expensive than tricaine, water soluble and needs to be buffered as tricaine. Its advantages are rapid induction and recovery. Reported to a gill irritant. Although fish lose equilibrium, there is not a total loss of reflex response.

2-phenoxyethanol (phenyl cellosolve) used at a rate of 0.1 - 0.5 ml / litre. It has a narrow margin of safety. Although cheaper than many anaesthetics it has adverse side effects such as a long induction time and erratic swimming. It may cause liver and kidney damage

There are several other agents used, but these are not readily available. The other option available to hobbyists in some areas are proprietary preparations. For example in the UK, NT Labs now market Koi Calm

how to use anaesthetics

There is a risk

At the risk of becoming repetitive, it is important to remember that there is a risk involved when anaesthetizing fish and the procedure does require some knowledge and experience.

Avoid all unnecessary stress

Regardless of which anaesthetic agent is used, all possible steps should be taken to minimize stress on the fish being anaesthetized. Stress responses will increase the resistance to anaesthesia and therefore increase the dose required - which in turn will reduce the safety margin.

The fish should be caught as quickly as possible, handled as little as possible (ideally handled with a stocking-net), food should be withheld for up to 24 hours beforehand and the fish not disturbed prior to anaesthesia.

The anaesthesia vessel should be prepared beforehand containing water that is the same pH, temperature, salinity and hardness as the fish is used to (i.e. ideally using water from the pond or tank). The water should be well aerated before and during sedation. Recovery tanks or water should also be prepared before use, again being well aerated.

There is a considerable variation in the response to anaesthetics which can result from individual metabolic differences, temperatures and physical condition. Unless you are absolutely certain of the correct dose it is best to put the fish in the induction tank and slowly add the anaesthetic agent - starting with a low dose. By careful observation you will minimised the possibility of an overdose. It is important to observe the fish at all times and if necessary terminate the induction. The aim is to achieve loss of equilibrium within 2-4 minutes. If the fish has still not succumb in that period, additional anaesthetic should be used.

Potential dangers

The aim for most procedures, such as scrapes and injections is to produce a state of light anaesthesia (a deeper state may be required if the fish requires a lot of attention). What we don't want is either for the fish to die from an overdose - but equally we don't want the fish to revive while we are injecting or taking biopsies.

Some experience is required to gauge when the fish is sufficiently anaesthetized and ready to be removed from the water. As a very rough rule of thumb, this would normally be between 30 -60 seconds after the fish has lost equilibrium and ceased movement.

There are three common potential dangers when using anaesthetic:

  • Fish that become highly stressed may start to haemorrhage from the gills during induction. This can vary from slight to profuse bleeding. Fish that bleed often have problems recovering and may fail to regain consciousness. The risk is related to the severity of the haemorrhage. I suspect that this may be due to congealed blood clogging the gills. In this situation it may be necessary to terminate the induction and try again the following day; or alternatively carry out the procedure as quickly as possible without removing the fish from the anaesthetic vessel and then place the fish straight into the recovery tank. Keeping the fish submerged seems to reduce the amount of congealed blood clogging the gills. Obviously this can only be done if the procedure can be carried out in a matter of seconds, for example an injections or scrape, otherwise there is a risk of overdosing
  • Where it is suspected that the fish may have gill damage, be very carefully as the fish may have problems recovering. Typical signs are heavy and laboured "breathing" judged by operculum (gill cover) movements
  • Some fish fight the anaesthetic and take considerably longer to lose equilibrium than is normal. Such fish will repeatedly turn on their side as is normal, but then fight and regain an upright position - repeating this several times before finally succumbing. The net result is that such fish receive a higher than normal dose and can have serious problems recovering. In these cases, it is important to carry out the procedure as quickly as possible so that the fish can be returned to water for recovery.

Induction and recovery times are usually increased at lower temperatures and therefore the margin of safety is reduced

Quick as possible

The tank immersion method is good for procedures that only require the fish to be out of water for a minute or two. If the fish needs to be out of the water for more than a few minutes, a constant recalculation system should be used. This involves aerated water containing anaesthetic being pumped through the fish's mouth and over the gills.

Examining the fish

It is important that the fish is handled safely and no damage is done to the integument (skin). A baby's changing mat makes the ideal surface to examine a fish. The plastic surface ensures that the mucus is not stripped off and if the fish does start thrashing around, the padding helps prevent damage. And of course they are easy to wipe down and keep clean. A wetted towel is often recommended; these are not ideal as the [relatively] rough surface of the towel tends to remove mucus.


In most cases the fish will be treated near to the tank or pond and can be returned to its home for recovery. If this is not the case a well-aerated recovery tank should be prepared in advance.

Recovery is straightforward in most cases, with the fish either being held just below the water surface or placed back in a net or shallow area where its progress can be monitored.

One would expect to see slight signs of operculum or jaw movement almost immediately. If there is no sign of recovery within a minute or so, it will be necessary to try and get water moving over the gill filaments. This can be done by opening the fish's mouth (by pressing gently under the chin) and moving the fish slowly back and forth, preferably near an air stone. Alternatively and preferably, the fish can be held (with its mouth open) in the water stream of a venturi, which will force oxygenated water over the gills. Both of these methods will aid recovery.

In extreme cases it can take up to 15 minutes to revive a fish that has been overdosed - so don't give up.

The fish should be monitored for 24-hours for any adverse effects.

Anaesthetics for use in euthanasia

When a condition is untreatable

As with all other animals, there are situations when the fish, for a variety of reasons, either fails to respond to treatment or has a condition that is untreatable. It is my belief that fish are capable of suffering and therefore the most humane act in such a situation is to euthanase the fish.

Getting up on my soapbox - euthanasia should be a last resort and not simply a way to save on treatment.

Any form of euthanasia should cause death with the minimum of discomfort or suffering to the animal. Typical methods used are:

Anaesthetic overdose: Use 3-4 times the recommended dosage of any of the commonly used anaesthesia agents. Because of the risk of recovery the fish should be left in the solution for several hours.

Ideally, after losing consciousness in the anaesthetic solution the fish should have an injection of sodium pentobarbital so that death is instantaneous - but this is a restricted drug and not available to members of the public. An alternative is to cut all the gill filaments on both sides once the fish is unconscious and allow the fish to bleed to death. Clearly this can be distressing, but the fish feels no pain.

Decapitation: This is only suitable for smaller fish. It is assumed to cause instant unconsciousness.

Cranial concussion: A sharp, single blow to the head with enough force to damage the central nervous system. This may kill small fish outright, but larger fish may only be stunned - so it should be followed by a method such as bleeding, to cause death

Sodium bicarbonate (baking powder or Alka Seltzer®): This is not an accepted form of euthanasia and will cause a degree of discomfort and stress, but it is readily available and can be used in the home if there are no other alternatives available. Again bleeding may be required once the fish has lost consciousness.

Freezing: This is not considered a humane method and will cause considerable suffering

Poisoning: There are many chemicals that will effectively kill fish. None are considered humane and will all cause considerable distress unless the fish is euthanased first

Frank Prince-Iles

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