Fish anaesthesia with Aqua-Sed

Fish anaesthesia with Aqua-Sed Fish anesthesia with Aqua-Sed

Before you anaesthetise a fish you should check the fish using the checklist below:

-    Fish should be viewed in the water (from above and from the  side if possible), assessing their movements and respiratory rate. In general, fish movements are fluid and seem relatively effortless.

-   The respiratory movements are generally slow and not obvious. Wooden movements, or marked respiratory movements, should be viewed with suspicion. 

-   The skin in general should be smooth and unbroken; haemorrhages may suggest localised scale damage or more serious septicaemic problems.

-   The scales should normally lay flat against the body. Lifting of scales to produce a ‘pine-cone’ effect may be due to localised or generalised fluid build up in the skin, or due to abdominal swelling due to ascites (fluid in the belly of the fish).

-   During the overall assessment of the condition of the fish including observation of its respiratory action, gills may be glimpsed partially. In the majority of cases it will be necessary to lightly anaesthetise fish to examine the gills in detail. The gill can be seen by gently lifting the operculum and using a suitable light source to illuminate the buccal cavity. The gills should normally be a healthy ‘salmon pink’ colour, with clearly demarcated primary lamellae.


Anaesthetising fish is fairly easy but still isn’t something to be done lightly. Any anaesthetic carries a risk, sometimes because the fish has an unknown/undiagnosed problem which can cause an issue. For this reason we don't suggest the routine use of anaesthesia of koi carp just to give them a general check-up. However there are instances where anaesthesia is extremely useful, such as:

- surgery/debridement

- handling of valuable fish (particularly large fish)

- close examination of a fish with a problem

The needs of these various tasks are different so care must be taken with all applications. The transition between stages of anaesthesia can be very rapid so great care is needed. The individual response of a fish to an anaesthetic and its transition between the various stages is dependent on a number of factors.

     -   Species

     -   Gill area to body weight ratio is particularly important

     -   Size and weight: metabolic rate

     -   Fat (lipid) content which varies with season of the year

     -   Sex, maturity, diet, condition, disease


Stages of anaesthesia

Stage 1    Light sedation, slight loss of reactivity

Stage 2    Deep sedation, total loss of reactivity except to strong pressure, equilibrium normal

Stage 3    Partial loss of  erratic swimming, increased gill movements, equilibrium            

Stage 4    Total loss of reactivity only to deep pressure, equilibrium

Stage 5    Loss of reflex total loss of reactivity, very shallow activity, opercular movements

Stage 6    Medullary gasping followed by gill movements stopping

Golden rules for Anaesthesia

-   Take your time and don’t try to hurry the process, especially with a species that you haven’t anaesthetised before.

-   Plan it properly.

-   Anaesthetised fish must be watched throughout the procedure.

-   Don’t take them deeper than necessary, inevitably the level of anaesthesia will deepen a little when the fish is removed from anaesthetic due to the drug being absorbed but not yet ‘hitting the spot’. It will rarely be necessary to go deeper than stage 3-4 above.

-   When you take the fish out of water for a procedure, lay it on a wet towel and cover the eyes and tail with the towel, ideally have assistance who can 'control' the fish while you do what is necessary.

-   Have your container of fresh water available for recovery. It's often safer and easier to use containers for recovery rather than simply immediately returning fish to the pond.


Vetark Aqua-Sed is a phenoxethanol anaesthetic. It's a liquid with a very wide safety margin and has been used for anaesthetising fish for many years (reports go back to 1943!). Aqua-Sed presents a significant leap forward for companion fish welfare; it is also approved for the euthanasia of fish. The label carries instructions on how to humanely euthanase companion fish, if using for this purpose use 4 times the anaesthesia dose.

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