interesting debate, & until more research is done it will remain just that.
Some thoughts
Pain/pain response.... the debate as to whether animals feel pain is a typical anthropogenic one... ie we try to equate stimuli & response mode to a human stimuli/response mode, carried to even a basic logical conclusion there is no way to show any creature other than man "feels pain", as , as has already been said "they cannot tell us it really hurt". As a guide I would say that if a response to a stimulus produces a result analogous to a pain response in humans than the animal can be said to respond to "pain". At the end of the day I think it can be argued that humans merely react to unpleasant or potentially harmful stimuli in a way we call "pain response" and that it is inseparable from a similar action in other animals ( any that would react in a predictable way to a given stimulus). Just because we can give it a name and communicate that to other humans is by no means a scientific method of differentiation, as we are totally unaware if other animals can effectively communicate similar information, just because they cannot communicate such info to US does not mean that it does not happen. (OK a slightly wider application than just fish here)
Research to date
Almost all research done in aquatics, and especially in medications etc are carried out on commercial fish.(after all that is where the money is) Therefore they are usually much larger and predominantly cold water species. It does not follow that such research is applicable to much smaller and tropical species. However it can be used as a baseline for further investigation. When it comes to tropical species held in captivity it is likely that most "useful" info will come from serious hobbyists and commercial breeders rather than scientific institutions..... AFAIK there is no "Rockefeller guppy institute", or similar conducting extensive and in depth research into ornamental tropical species. Commercial fish farms may commission research into specific problems, but IME this seems to be more a case of " get a vet & try to fix it", rather than a stricter sense of scientific research. The findings of such an investigation are no less valid for not being published or reviewed, but also should not be treated as "fact" until verified.
Clove oil as an anaesthetic
used by US fisheries for documented investigations into non food species. Also used worldwide as a widely available "non vet" anaesthetic, used in several proprietary medications for this purpose.
Clove oil for euthanasia
the attached report seems to indicate it's unacceptability based on lack of research or available verifiable data. Such research is unlikely to be forthcoming as clove oil is not a protectable ingredient under patent or copyright, and therefore there is no commercial gain to be made by further research. Products such as MS222 are commercially viable as the product can and is patentable and protected by the manufacturer, therefore commercial research is also viable.
My experience in euthanising fish from 1/2 to 16" is that both ms222 and eugenol (clove oil) are equally effective. as clove oil is easily obtainable I will restrict further comments to that product alone. Fish exhibit no "stress response" ... trying to escape, thrashing round, rapid movement or breathing... at sub lethal and lethal doses. Fish subject to sub lethal dose recover rapidly in clean water and exhibit no signs of ordeal following the useage, I have found no long term problems in its use as a calming agent or anaesthetic, lifespan does not seem influenced. (please bear in mind the objective nature of that statement and the small sample used). Fish subject to a lethal dose pass away quietly and without fuss.
I would generally agree with the findings of the report regarding other methods of euthanasia
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