Anaesthetics 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. <= DO NOT DO THIS: IT IS LEFT HERE AS A WARNING. I HAVE SEEN PEOPLE EXPLODE THE GASTROINTESTINAL TRACTS OF KOI WITH PRESSURIZED VENTILATION. 

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.