Broaching Difficult Aquarist Issues

I usually just set the tank up saying 'don't worry it's only temporary', then somehow forget to take it away and move other fish in there instead. But that's not likely to work here 

I ventured back into the fish room later on yesterday evening and put the dead/almost-dead fish back in the main tank where her/his body fell into a plant.  I felt that the least I could do was let him pass away definitively (if he hadn't already done so) with his tankmates, but couldn't make eye contact with any of them.
I then spent an entirely sleepless night, racked with guilt, and coming to the realisation that while I love fishkeeping and care deeply for the fish and snails in my care, and give them the best life I possibly can, I fail completely on the capturing/netting fish aspect which is a key component of fishkeeping and therefore I couldn't continue in the hobby.  12 hours later, I was still purple-faced with guilt - it was as though I'd induced a case of scarlet fever as I've never had such intensely flushed/purple cheeks sustained for so long.
Imagine my utter confusion when I dared peek in and noticed all 7 cardinals were swimming this morning, including the 'dead' one.

Photos show how he was yesterday and hence why I intended to medicate - he wasn't going to survive whatever this is without an intervention and his tail looked at risk of dropping off, yet he seemed constitutionally capable of withstanding treatment and so I wanted to give him the opportunity of survival. However, I must have induced a stroke/ heart attack/ similar with the trauma of attempted capture. He'll probably die imminently from the former, or the combination, but I suppose a small percentage of my guilt is reduced by it not being immediate. In this case, I'll have to leave whatever happens to nature.

I must admit it is nice seeing the plants thriving in your picture even is the fish in question is playing games with your nerves. I do sometime feel that I must be absolutely deficient at catching fish when I see the people in the shop seemingly do it with such ease. On my last visit I had the realisation that their nets are significantly bigger than mine and so I am thinking I might try the herd the fish into one net using the other technique next time. It’s always stressful for all involved though.

Sound like your fish has some fight left in it at least! Fingers crossed for you both

As an update, this fish lives on (touchwood).  Its caudal peduncle is white/raw, and intermittently develops 1-3 white pustular spots on it.  I add Melafix to the tank when this arises.  Whether or not they would subside of their own accord, or the Melafix is effective in these circumstances, I don't know - but, for now, this seems a suitable course of action.  The fish is happy otherwise - chasing down food, sociable, etc.

I am so sorry it came to this, fcmf  But there does come a time when it's the kindest thing to do - but even then it's not an easy decision.