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Exploring Fish Euthanasia

Some of the methods described in this article are dangerous to life and limb and should not be attempted by the incompetent - this article is for information purposes only, and as such no liability will be accepted for any mishaps caused in the replication of the descriptions.

Euthanasia n. an easy mode of death: the act or practice of putting painlessly to death, esp. in cases of incurable suffering.
(Chambers English Dictionary)

Having to kill a fish is something that most of us do not relish - it's very different to kill a fish that you have caught to eat than it is to put an end to the life of a fish that you have cared for over a period of time.

Unfortunately fish get ill or injured, and fish get old, sometimes in the process to such a degree that their suffering becomes an affront to us and we feel that something must be. In the best case scenario fish just drop dead, floating on the surface in the morning having died peacefully in the night ( the way that I image Forest Bump's fish might pass away), more often than not, however, you notice the fish becoming weaker and and in difficulty as a process, such as death is.

This article cannot help you decide at what stage to euthanise your fish, although fish do feel pain it is entirely subjective - they do have the same reactions to pain as we do, but are incapable of expressing it; therefore you should ensure their death is as painless and sudden as you would wish your own. This article looks at the most commonly used methods and their impact on the fish before it eventually dies. All fish should be given a fair chance at recovery before the decision is taken to kill it.

Each method will be looked at in relation to the definition of euthanasia - and rated on how much or little suffering the method will cause on top of the underlying terminal problem.


This is the method coveted by the ignorant and cowardly. A sick or injured fish is flushed down the toilet to die in the sewer.

I have heard justifications of this such as "Once it's swirled about a bit, it'll be dead anyway" - this is not true in any way. Fish are aquatic animals and are generally used to water current.

In the best case scenario the fish will die from temperature shock quite quickly compared to the prolonged suffocation and burning caused by chlorinated water, cleaning products and effluent.

This method should never be employed - out of sight, out of mind does not equate to a painless death.

This method is not recommended for any fish.

Humane rating: 0% (if I could use a minus figure I would)

Sudden blunt trauma

This is most commonly referred to as "whacking with a hammer", or brick, or angler's priest. The end result is the same - crushing the fish's skull to kill it.

The method involves taking the fish out of the aquarium, laying it against a sturdy surface and then dispatching it with the blunt weapon of your choice - some notables being sledgehammer and brick.

By the very definition of euthanasia this method is not completely humane - the fish has to be handled which causes stress, it has to be taken out of the water, which causes stress, and it has to be laid on the ground (or whatever surface) which also causes stress - this is all exacerbated by the amount of time it takes for you to line up your 'shot'.

An alternative to laying the fish down to hit it with an object is to hold the fish by the tail and swing it against a hard surface head first - this may not be successful on the first attempt however and as such is not recommended.

Although the actual death is quick (so long as you get it right first time) the process can be convoluted which comes under the heading of cruel and unusual.

Recommended for: Large fish that cannot be accommodated in any other way.

Not recommended for: Small fish, or those likely to "flap about a bit"

Humane rating: 60%


Similar to sudden blunt trauma, but the fish is dispatched by removing the fish's head just behind the gills using a sharp knife, or other bladed instrument.

The detractions to this method are the same as sudden blunt trauma - handling, removal from water and laying on flat surface all increase stress before the eventual death. In small fish you can use a sharp pair of scissors once removed form the water.

Recommended for: any fish, as long as you can successfully decapitate in one movement

Not recommended for: fish that are still strong enough to flap about.

Humane rating: 50% (this is due to evidence that brain activity continues after 'clean' decapitation for up to 10 seconds)

The blender

This method involves placing the terminally ill fish into a blender and "whizzing" it. The main problem with this method is that the blender should be running before the fish is introduced (head first) to ensure that the fish doesn't fall below the blades or become impaled.

Again, you have to catch the fish, handle it and then dispatch it. Stress levels would become quite high.

Recommended for: Any fish whose head is not deeper than the distance from the top of the blender blades to the bottom of the blender jug.

Not recommended for: Any fish that does not fit in the blender - fully grown iridescent sharks, freshwater rays, you get the idea.

Humane rating: 90% if the blender is already running, 30% if you have to turn it on after putting the fish in.

Freezing method

The idea behind this is that the fish goes into a dormancy mode as the temperature decreases. The fish falls asleep and never wakes up.

The method is you place the fish in a coverall container of tank water at the normal temperature - this is then covered to simulate dark and placed in the freezer.

I am divided on this method and I think that the humane factor depends on the species of fish - I know for a fact that when Carassius auratus and other members of the carp family are exposed to temperatures lower than 55F they do indeed enter a sort of dormancy, their metabolism slows and indicate sleep behaviour. This does not indicate that all species enter this state - lower temperatures spontaneously create diseases like ich in some species.

Also the temperature drop in a freezer is quite rapid (the temperature will drop roughly 1 degree for every five minutes in the freezer) unless you were to place several gallons of water to freeze. We have acted in the time that we have cared for the fish to protect it from rapid temperature fluctuations and yet are now doing the very thing that we have tried to avoid. Temperature changes such as described above will produce stress until death, or the dormancy stage is accomplished (if the fish is stressed there is no guarantee that it will enter the dormancy stage - a human that falls asleep on a freezing hillside will wake up before freezing death and only loses consciousness as the body's homeostatic system constricts bloodflow to the extremities (including the brain) to keep the core warm, there is no reason why the same shouldn't happen in fish).

Recommended for: tenuously recommended for cold water members of the carp family and any other species that has a proven dormancy stage

Not recommended for: tropical fish

Humane rating: 85% (for carp family) 25% (for tropicals)

Temperature shock method (freezing)

This method utilises the coldest water you can find - a bowl of water that has been placed in the freezer long enough for the surface to freeze is good. The surface ice is broken and the bowl taken to the dying fish.

The fish is then captured and dropped into the freezing water. The difference between the temperature of the tank and the freezing water causes an instant fatal shock to the fish nervous system stopping all organ functions instantaneously - anyone who has found themselves suddenly in water that is much colder than your body temperature will know the feeling when you initially can't breathe (people have died of heart failure under similar circumstances), it is like that but much more severe and sudden.

A variation on this is freezing alcohol (pure or vodka)

Stress is caused in the handling of the fish, but in most of the methods the fish must be handled - however this handling is brief.

Recommended for: tropical fish

Not recommended for: coldwater fish

Humane rating: 100%

Temperature shock method (boiling)

Works in the same way as the above temperature shock method, but involves dropping the fish into boiling water.

The main problem with this is not the effect on the fish, but rather our perception of it. Boiling something is usually for food, and things that are boiled are usually dead (lobster being the exception and falls under this category).

However this method is completely acceptable and humane for coldwater fish.

Recommended for: coldwater fish

Not recommended for: tropical fish

Humane rating: 100%

N.B. Caution should always be exercised when working with hot liquids, surfaces and devices.


As pure as possible - vodka, tequila.

The method is that you have the alcohol at the same temperature as the water in your aquarium - the fish is introduced to the alcohol and immediately becomes overwhelmed by it.

Alcohol works like an anesthetic and is at lethal levels. The fish will instantly be paralysed and unconscious, death follows quickly due to respiratory failure.

An alternative is freezing alcohol as mentioned above - that might, just be overkill.

Recommended for: all fish

Not recommended for: fish who attend alcoholics anonymous

Humane rating: 100%

MS-222 (or other veterinarian anesthetics)

This is possibly the Rolls Royce of fish euthanasia.

Like all anesthetics, if you administer too much death occurs. This is particularly useful if the fish is solitary in a quarantine tank. You can make a bath of it, which is the most economical way, but for minimal stress you should use it straight into the tank.

MS-222 is a white powder and should be mixed with water prior to use. The MS-222 solution is then poured into the tank water. Death occurs in the same way as alcohol above.

Recommended for: all fish

Not recommended for: N/A

Humane rating: 100%

N.B. MS-222 and other veterinarian anesthetics should be handles with care and stored securely.


This method is dangerous - it should not be attempted by unqualified individuals!

This method involves placing negative and positive electrodes in the aquarium water and producing a charge sufficient to kill the fish.

The key to this method is amperage. It is the amperes of a current that kill, not the voltage. Volts is another word for the kick, while amperage is the power in real terms.

Death occurs due to cardiac arrest when accomplished properly.

Recommended for: all fish

Not recommended for: anyone who doesn't know what they are doing.

Humane rating: 100% (when done properly)

This article and the text contained within are ©David Nicol 2001 - All rights reserved. Permission is granted for distribution of article for personal use only.

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