Euthanasia is not at all pleasant. I don't like doing it, and certainly strive to try whatever I can to keep fish alive - fortunately, I managed for several months to keep one elderly and very disabled fish alive earlier this year (in an isolation tank and with fish-fry food) who seemed to be at death's door in the main tank, and it was a very rewarding experience. Unfortunately, though, he did start writhing/rolling around on the bottom of the tank one day, and there was no option at that point. :'(
Re: What is the Most Humane Way to Euthanize a Fish?
It's a difficult one. On a continuum, I tend to be much more on the side of "where there's life, there's hope". In my experience, I've had all sorts: a goldfish that seemed to be at death's door for quite some time but somehow, and including medication, made a complete turnaround and went on to live for several more years; the same goldfish that was seemingly dead but showed signs of life every so often over a 22-hour period before ultimately dying; elderly fish that developed increasingly bent or seemingly-broken spines and struggled or lost function of their swimbladder for spells of time throughout the day but that I've managed to keep alive either in the main tank or in an isolation tank where they struggled less and did not cause stress to other tankmates; etc. Eg: * In most of those cases, I'm certainly glad that I didn't euthanase (others would have), as I feel that the fish still maintained a good quality of life and taught me to be innovative with adjustments for them to maintain that life while minimising impact on others. One fish had been hanging at the surface "playing dead" but perked up when transferred to an isolation tank with a gentler filter outflow and was able to begin eating again with baby/fish-fry food. When I did ultimately euthanase any of these fish, it was weeks/months later when within an hour of death (eg rolling around on the substrate without reprieve, complete loss of swimbladder function over several weeks and then ultimately hanging at the surface by its tail to the point of tankmates inquisitively poking it and no longer seeming conscious of its surroundings or my presence). * In some cases (eg my goldfish on the second occasion), in hindsight, perhaps I ought to have euthanased - but wasn't certain or experienced enough with fish deaths at the time to take that action. * In other cases, I did euthanase, but twice regretted it - the fish definitely was at death's door and would have died within an hour (rolling around on the substrate, etc), but the method used on those occasions (Aqua-Sed rather than clove oil which I've probably used 2 and 5 times respectively) was far more traumatic and the fish seemed to temporarily resurrect and attempt to escape as though drowning - I would absolutely never use it again, but NB others speak very highly of it and prefer it to clove oil. The 5 occasions with clove oil I've not regretted in terms of relieving undue ongoing suffering and thankfully it was swift and less than 5 seconds - but I did find it very upsetting to do and it does cause inner guilt/moral/conscience issues, which is why I'm always slightly relieved if a fish has died naturally.