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
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
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
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
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
how to use
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
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.
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
- 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
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