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I do understand that with a tetra or molly for example it could be rather interesting to try that method, and common sense should play an important role if anyone does decide that killing a fish is the only method left to them.
Apart from my work in the labs and fishing I haven't and probably(touch wood) wont kill any fish, as quite a lot of things can be treated. The papers are readily available in the journal of veterinary medicine and the journal of Aquaculture however these are not usually available free of charge I'm afraid but for those who are that way inclined a subscription to either of these journals would give anyone with a scientific inclination more than enough information on fish biology and medicine.

Thanks for the link Asumel. It makes an interesting read. I would still have great concerns about recommending that method to anyone not experienced in handling fish of different sizes. Below is a link (not pleasant) which shows the likely result of inexperience.

http://www.youtube.com/watch?v=U4OMd_xPoYE&

Not to mention how the more squeamish would cope, I think the freezer method is still advocated in some circles more for squeamish owners rather than the interests of the fish and I think decapitation of an old friend may send some back to that method. Because of these concerns I still think that for pet fish the oil of cloves method is the best choice.

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It was Cathie who deserves the credit for the link, so I cant take credit for it.
That link is an unfortunate example of someone who is rather inexperienced and doesn't appear to care for the welfare of the fish.
The method me and my colleagues use is to catch the fish in a net, and conducting the killing while its still in the net, this prevents inaccuracy (Like that in the video)ect
Having seen the data for the other methods this is the method that I would prefer to use.
I would like to make it clear now that I would NOT kill my fish unless there was no other option and it faced prolonged suffering

I and the institution that I work with pride ourselves on the highest fish welfare possible, putting the health and wellbeing of the fish before scientific and monetary gains. There is NO value, scientific or monetary in keeping and working with fish who are not well cared for.

Sorry Cathie, and thank you for that link.

I have had need to euthanise fish in the past as a last resort and to prevent unnecessary suffering. Sooner or later that situation will face all fish keepers.

I made a video using the phenoxyethonol2 sedation method

http://www.youtube.com/watch?v=4ww4SQYAuQ8

The fish in the video was actually just sedated in order to treat a facial ulcer but the video demonstrates the effectiveness of the method and if used at an higher dose and the fish left in there then death is guaranteed and in my view relatively stress free. (The fish in the clip went on to make a full recovery and is alive and well today.

Which ever method is used I would urge people not to hang on for to long waiting for a miracle recovery which is never going to happen, it is important that pain and suffering are stopped as quickly as possible.

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I completely agree with this statement, and would like to add that any method used to kill fish should not be conducted if the conductor is not comfortable with the method as this can lead to hesitation and can prolong the suffering of the fish.

Unfortunately, I was faced with a cull of whole species only set up of Neon Tetra tank following a mass worm infection that had migrated to the outer flanks and eyes. This was a few years ago, when none of the meds I tried were effective

As distressing as it was, clove oil came to the rescue and it was very peaceful and quite quick. Some larger fish it has been reported show distress for the first few seconds but given decapitation of 80 or so Neons, would be undoable, I favoured this option.

Freezer is barbaric IMO, so too flushing or boiling (yup, seen that last one there on another forum)

Nathan Hill wrote an excellent detailed article on the pros and cons of the various methods used to euthanize fish. Reading through that again after seeing this thread I would urge anyone who needs to do this (and we all face it sooner or later) to keep either a bottle of Aquased (2-phenoxyethanol) or a bottle of clove oil in the fish first aid kit.

Many of the commonly recommended methods are NOT a pleasant way for the fish to die and particularly in unskilled hands. You can read the original article in full on the PFK web site here, but below are just a few reasons why some of these methods are not the best option:

Ice - can be used for very small fish (up to 5cm but must be done correctly. Fish are cooled in water with crushed ice added, but this must be done slowly enough to prevent ice crystals from forming in the fish's body before death which would be extremely painful. The ice must not come into contact with the fish and fish should never be placed directly into a freezer (as some recommend).

CO2 overdose/Alka-Seltzer - This is very unpleasant for the fish, so much so that it has been banned as a method of slaughter for commercial food fish production in the EU. Should not be used.

Boiling - this shouldn't need explaining, but as this one does the rounds still, it should never be used. Death is slow and extremely painful.

Hitting the head - can be quick and painless, but only if you are confident enough and capable enough to succeed with the very first blow. Also consider that once the fish is out of water you need to act immediately to prevent stress from that situation. If you're squeamish, nervous or at all unsure, don't attempt this.

Decapitation - Must be achieved with one clean cut and immediately followed by pithing: insertion of a sharp object into the brain cavity to destroy the brain. If this isn't done the fish can remain conscious for some time after decapitation.

Flushing - extremely unpleasant for the fish. It will be subjected to the trauma of being netted, then a sudden and extreme change of water parameters and temperature. Many fish will still be alive when they hit the sewage system and then die in human sewage. Some even survive and end up in the waterways which is why it's also legally a questionable practice.

Other methods are also covered in the PFK article, but suffice it to say for the majority of us an anaesthetic overdose is the least traumatic and most peaceful way to say goodbye to a fishy friend.

PFK have recently published a new article about some of the chemical methods of euthanasia, well worth a read and it reaffirms that clove oil is currently the best method available

http://www.practicalfishkeeping.co.uk/content.php?sid=6288

I heard u should freeze it

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Some people believe that's the best way, but it is far from humane. Ice crystals form in the fish's body, causing a very slow, uncomfortable death.