One of the ironies of the fishkeeping hobby is that despite the care frequently lavished on our pets, relatively little advice exists about the best ways to kill fish painlessly should that be necessary. Many of the traditional methods such as the use of ice or Alka-Seltzer are notably absent from the list of euthanasia techniques available to scientists (Borski and Hodson, 2003; Brown, 2005), and in some cases are strongly criticised by experienced aquarists and vets as being inhumane (Burgess et al, 1998).
Why euthanise your fish?
Euthanasia is a way to relieving the suffering where the fish is so grievously injured or so seriously ill that treatment will not help and recovery is unlikely. When fish reach this condition, letting "nature take its course" actually means condemning a fish to a period of suffering for an indeterminate length of time. In other words, even though killing a fish might seem unpleasant, it is actually a way of treating animals humanely.
When to euthanise your fish
Deciding when to euthanise a fish is appropriate is a difficult decision. Some diseases are incurable and highly contagious (for example Neon Tetra Disease or Dwarf Gourami Iridovirus) and consequently any fish suffering from these diseases are obvious candidates for euthanasia. Euthanasia may also be relevant for those fish that have been so severely injured that they will not heal and cannot be expected to live a normal life.
However, it's also worth considering situations where euthanasia isn't appropriate. Most diseases fish suffer from in aquaria are quite easily treated using a variety of inexpensive medications. Chronic diseases such as lymphocystis and fish pox may not be easily treatable, but they don't tend to cause suffering either, and given good water quality and a healthy diet, these diseases may fade away in time. There is usually no need to euthanise unwanted fish either: these can normally be passed onto other aquarists, either through your local tropical fish store or else by getting in direct contact with other hobbyists via clubs and internet forums.
How to euthanise your fish: Anaesthetic Overdose
Vets consistently recommend that the safest and most painless way to destroy a fish is through administering an anaesthetic overdose of some kind (Andrews et al, 1988). With large fish such as oscars, marine angelfish and koi this advice is difficult to argue with, and anyone facing the prospect of euthanising large fish should contain their veterinarian. Under some situations it may be better to administer the anaesthetic at home because moving the fish will only stress it further.
Among the anaesthetics used for euthanising fish include:
In some cases these drugs will take a long time to work, as long as 30 minutes in some situations (Brown, 2005), and death should always be verified before the fish is removed, for example by checking that there are no gill movements within a period of 3 minutes (Brown, 2005). Burgess et al (1998) recommend leaving fish in anaesthetic baths for two or more hours to be sure.
Tricaine methanesulphonate (widely known as MS222) is acidic, and should be used on conjunction with a buffering agent if the anaesthetising solution has a different pH to the aquarium.
How to euthanise your fish: CO2
Carbon dioxide (CO2) can be used to painlessly kill fish. CO2 from a cylinder needs to be vigourously bubbled through the water for at least 30 seconds and then the fish placed into the container and left for at least 10 minutes to ensure breathing has stopped (Brown, 2005). Bottled CO2 can be easily and inexpensively obtained from various sources, not least of which is aquarium shops specialising in CO2 fertilisations systems for planted freshwater aquaria.
Note that this method is not equivalent to using carbonated water or Alka-Seltzer; neither of these alternatives appear to have been evaluated by the biological or veterinarian community and cannot therefore be recommended.
How to euthanise your fish: Clove oil
Clove oil, also known as eugenol, is widely sold as a local anaesthetic, for example to treat toothache. It has also been advocated by some researchers as being a safe and inexpensive sedative for small fish including characins and cyprinids (Roubach et al, 2002; Grush et al, 2004). It should be noted though that at least some components of clove oil are carcinogenic (Borski and Hodson, 2003), so care should be taken when handling this chemical.
Clove oil can be used to euthanise fish because it causes fish to rapidly lose consciousness and quickly induces hypoxia, both of which reduce pain (Borski and Hodson, 2003). The required concentration will need to be quite high though; Borski and Hodson (2003) recommend at least 400 mg/l, with the fish being exposed to the solution for at least ten minutes after the cessation of gill movements.
At home, try this method: Put a litre of aquarium water in a plastic container, and then add 30 drops of clove oil. Mix thoroughly. Place the fish in this mixture, holding down with a net if necessary, particularly if the fish is able to breathe air (if it could breathe air while in the clove oil solution it would take longer to die). Place a towel or similar over the container to stop the fish jumping out. In the author's experience with fish will be sedated within 30 seconds to a minute, effectively comatose within ten minutes, and, in the case of small to medium-sized fish, and dead within half an hour.
How to euthanise your fish: Decapitation and concussion
Although messy, stunning and then decapitating a fish will painlessly kill it, though a process called pithing (where the brain is physically destroyed after decapitation) is required because fish can remain conscious even after decapitation (Burgess et al, 1998). Pithing is done by inserting a metal rod (such as a skewer) into the depression just above and behind the eyes through the brain and spinal cord, so that the central nervous system is completely destroyed.
How to euthanise your fish: Freezing
Burgess et all (1998) do not recommend the use of ice or freezing water to euthanise fish, but Brown (2005) specifically describes a process that is acceptable for very small fish (under 5 cm in length). The fish is either (a) exposed to freezing cold water but is not in contact with the ice itself; or (b) placed in a container of normal temperature water from the tank and then that container is transferred to a freezer where it can cool down to below freezing.
How NOT to euthanise your fish
Burgess et all (1998) site the following methods as being unacceptable because of the cruelty involved:
How to dispose of the carcass
Once the fish is dead, the important thing is to make sure the body cannot contaminate local waterways, which is why flushing the body down the lavatory is not recommended (Burgess et al, 1998). Instead the body can be burned, buried in the garden, or disposed of with the household trash.
For many aquarists, losing a valued pet fish is a sad experience; this is especially the case where children are involved. It is always valuable to put the cycle of life and death into context. While fish are relatively long-lived animals, particularly when compared to mammals of similar weight, they don't live forever. Small fish such as tetras typically have a lifespan of up to 4 years, often less in the case of species that are effectively annuals in the wild. Medium-sized fish like damselfish and many cichlids will live for between 5-10 years if cared for properly, while bigger fish like plecs, oscars and lionfish should easily reach between 10-20 years. Coldwater fish are particularly long lived: the record for a goldfish is 43 years, while the oldest koi lived for well over 200 years!
On the other hand, most fish sickness comes down to problems with water quality, water chemistry, or diet. If your fish get sick, and especially if you find yourselves having to euthanise your fish, then you absolutely must focus on aquarium maintenance and try to establish what is going wrong. Sometimes the problems are obvious (detectable levels of ammonia and/or nitrite, for example) but in other cases you'll need to closely review the needs of your fish, cross-checking these demands against what you're providing in the aquarium.