Further update on Bendy Harley:
I've been putting the plant in the tank during the daytime when I can keep an eye on him and he's learned to live alongside it - but keep the plant in a jug with tankwater overnight when I can't keep an eye on him to ensure that he doesn't get tangled up in it.
Separately, yesterday, I noticed his tail was looking 'heavier'/paralysed and that he's now more nose-up and his swimbladder seemed to have started to deteriorate. He's also started some episodes of gentle spiralling. I think he's moved into the next stage of deterioration now, and may not make it to his 5th birthday in a fortnight's time.
In the space of 5 minutes, there were 3 episodes of gentle spiralling - the first two were difficult to watch but I managed to hold the video camera still to take the third episode: https:///youtu.be/CXYLbbpIKdY
Update 5 mins later:
He's completely upside down on the bottom now, gasping. It looks as though he's now at the point of no-return. Mercifully, this has been swift. Frustratingly, though, Furloughed Mr FCMF is having to make a conference call into work, so I can't be in the room - just keeping my fingers crossed that Bendy Harley doesn't slip away to the sound of loud voices rather than the respectful/calm send-off he deserves with me beside him metaphorically holding his fin! [Edited to add: I was beside myself being unable to be beside the tank, so entered the room under the pretext of bringing in a cup of tea; thankfully, Bendy Harley was more upright, so I gave him an encouraging nod (hopefully out of sight of the VC!) and which he acknowledged. At least the bellowing of the VC has reduced so, whatever has since happened, at least it's a more peaceful environment.]
Update 45 mins later:
Mr FCMF came out of the room, to retrieve a cable, and I had no option but to sneak in as discreetly as I could past the VC to check up on B.H. He was upside down on the bottom but intermittently breathing. I couldn't have him lying there with discussions about furlough/£/gov/redundancies in his final moments, so scooped him out in a jugful of water, tucked the E box under my arm, came through to the other room to have him in peace and quiet, and realised I had no option but to Do The Deed as he was upside down and only able to rectify himself intermittently for a few seconds.
I decided to do this in two parts. Again, I'm not convinced about this Aqua-Sed and think in future I'll stick with clove oil only. Suddenly he managed to rectify his position as though attempting to escape out nose first, and swam semi-upright for 15 secs in the A-S solution (rather than the ~3 sec short-and-swift clove oil experience). Although he seemed to be 'gone' after this, I carefully prepared the clove oil, and put it in too as a back-up. There seemed to be a momentary resurrection as he did a couple of circuits of the container, then fell again to the bottom like a stone. The container is now wrapped in a dark towel.
I'm worn out, in part as this relatively sudden demise after 2 months' stability was a bit of a shock, in part because of the E procedure (but felt there was no option), and generally as it feels like the culmination of many months running a palliative care home for elderly fish!
This now leaves me with the main tank containing what I hope is a relatively healthy community (touchwood!) containing one elderly harlequin, one neon green rasbora, five cardinals, two nerites, and some plants that may or may not survive depending on how soon the new LED light takes to arrive.