Elderly Fish

Sorry to hear about pop eyed X-rays deterioration. The potential for TB is intriguing... would this mean that you would need to allow your current fish to all pass and disinfect the tank and everything? I might do a bit of research myself to understand it a bit more...

In the absence of post-mortems, we simply don't know whether it's this or not but some other good points were made in the reply about moving any afflicted fish into the other tank, not restocking the main tank if in doubt about any inhabitants, no guarantee that any new stock were not carrying it, not doing tank maintenance if any cuts/grazes on hands, etc. Thanks for the reminder of the PFK article - I'll dig that out.  I think it'd also be wise to wear the gloves, irrespective of presence of cuts/grazes, so thanks for that.

In the main tank, I have the 5 cardinals (6.5 months since purchase), 1 neon green (1.5 years since purchase), 1 harlequin (4 yrs, 10.5 months since purchase). The spare tank has the harlequin that has the strange bendy behaviour and the deteriorated/last x-ray. In the main tank, the neon green has a slight curve/twist in her tail when viewed from above, and one cardinal has been breathing faster than usual for the past week or two. The neon green is quite hyperactive and would never survive the smaller spare tank, and I'm continuing to monitor the cardinal as I know they're particularly keen to stick around as a group so wouldn't want to move unless absolutely necessary.

Fingers crossed for your tetra.

Your fish are determined to hang on.......

I am sorry you had to do this. It is always hard making the decision.

End of an era @fcmf

Sorry to hear about this.

You fish really do reach a ripe old age in your excellent care and this was no different.

Thanks, both, for your kind words.

Fingers crossed for your little fella.

As an update to this thread:

* Bendy Harley's condition remains the same - stabilised in his isolation tank.  He spends the entire day in the "quiet"/current-free corner of the tank, observing the two humans and possibly the main tank.  He moves when food is dropped in, looks excited but I've never actually seen him consume it - but he must do at some stage, otherwise he would never have survived this many weeks (unless purely off the Hikari First Bites which float in the water column and would be ingested inadvertently).  Although his body is curved, particularly evident when he moves (it's always to the left), it doesn't generally affect him.  His seizure-like stiffening into the reverse C shape and plunging/catapulting in the tank, sometimes to the extent of falling onto his side on the tank substrate and then returning to the tank surface on his side or even belly-up, seems entirely related now to being startled rather than also due to falling into or inability to cope with the filter outflow as was the case in the main tank.  At night, after the room is in darkness, I have spotted him swimming about as though all is normal and the gentle filter outflow doesn't affect him at all - in fact, both he and Pop-eyed X-ray Tetra (RIP) seemed to liven up and move away from their respective static locations once the room was in darkness. I suspect he may even be swimming down to the bottom during the night and thereby getting food.

* I mentioned in another thread (https://forums.thinkfish.co.uk/the-emergency-room/double-emergency-room-sparkling-gourami's/msg48818/?topicseen#new reply #11) that Pop-eyed X-ray's final demise doesn't sit right with me. 
I do keep wondering whether the catastrophic demise was in some way a physical manifestation of a psychological / severe stress reaction from not being returned to the main tank when he started to improve (even although I thought best not to in case he had TB) - I'll never know the answer to whether this was a poor decision on my part and will just have to keep working on reconciling this in my mind. 
The other issue that still haunts me, and which may be of interest to others, was the Aqua-Sed use.  I've read extremely favourable comments about it elsewhere by a couple of folk, about it offering a much gentler 'departure' than clove oil.  Unfortunately, this was not my experience.  On the handful of occasions I've used clove oil, it has been swift - the fish has stiffened up / ceased swimming within 3-5 seconds.  With Aqua-Sed, unfortunately I witnessed Pop-eyed X-ray's head rising up out of the solution, looking as though he was gasping/drowning and attempting to escape.  It was difficult to see what happened after that as the solution was milkier than the clove oil / water solution but, on seeing the red tail continuing to swim below the surface, I couldn't bear it - and, in a panic, administered some drops of clove oil directly into the mixture (rather than pre-mixing it with water firstly - I just hope he was at least partially anaesthetised by this stage).
[Articles on the E issue are at https://www.practicalfishkeeping.co.uk/fishkeeping-news/painful-fish-deaths-you-might-be-guilty/ and https://www.practicalfishkeeping.co.uk/fishkeeping-news/time-to-rethink-anaesthetics/]

I can definitely see your rationale, although oddly the side he chooses to be in is the window/lightest side (but with least water flow) and actually his plummeting behaviour doesn't seem to be affected by bright lights - he can cope with the TV being on - but more so by people getting up from sitting or sudden movements or something being set down on the table the tank is on. He'll very happily "interact" with my face up close to the tank, talking to him. The more I think of it, the more this fits in with your "vibration" theory (although he's fine with music and voices).
For further security, covering the tank with a dark towel might indeed be a plan. I know that the last remaining harlequin in the main tank seems to look across towards Bendy Harley regularly as though keeping an eye on him - whether or not this is the case or not, I can't be certain, but perhaps I could reposition the tank in such a way that the front is covered but they can see one another through the smaller side and he can interact with me through the other smaller side.

Yes, the E topic is not one to be taken lightly - I'm horrified and disgusted at how lightly and readily some people elsewhere seem to consider it, such as early in the stage of disease or if a fish has a disability. My experience is that fish seem to adapt well to disabilities and my goldfish (RIP) had a near-death experience many years ago but made a complete turnaround and went on to live another few years following that (when others would have euthanased). Ultimately, though, his final demise took 20+ hours from seeming complete death (lying afloat on his side on top of the water for 20+ minutes, no gill movement, etc, interspersed with intermittent 'resurrections' of ~10 seconds, recurringly) to definite complete death; terrible to be in his presence during this but likely far more terrible for him to experience.
Even when the trajectory is clear in what irreversible direction it's heading, I definitely wouldn't agree with it - after all, we're all dying from the minute we're born but sometimes the path to that is swifter due to a specific condition. For me, I only use it when it's clear that the fish has only got minutes or under an hour or so to live and thus which prevents any undue lingering/suffering - and that knowledge comes with experience... usually it's rolling around or writhing on the bottom of the tank floor or, in the case of the elderly fish recently, a slow demise with a clear direction over many months culminating in hanging upside down and inability to maintain buoyancy at all from the tank surface (as distinct from a younger fish who might develop acute but treatable symptoms). Others, though, would have "done the deed" well before that point.
Bendy Harley is a good example of where he looked a "write-off" in the main tank (and likely would have died on his last day in there due to all the catapulting) but has gone on to live a peaceful existence in the isolation tank with the occasional episode per day. I have no idea what the future holds - I wouldn't be surprised to find him dead some morning but equally he could be around in 6 months' time. Wee Harley looks in optimum health but, at their advanced age (5 next month), anything could happen.
As for the actual administering of the E solution, I do find it an ordeal every time - distress and tears are always involved when I realise it's the only option, there's a delay while I gather my thoughts and/or provide an update on here (and hope for some form of divine intervention to turn the situation around), but I do it as respectfully as I can by having silence in the room or calm classical music at very low volume, have a "chat" with the fish to explain what's happening and thank them for their contribution to the tank and more-or-less deliver a eulogy of my memories of them, then talk calmly and reassuringly to them about what I'm doing as I administer the solution (although they can no doubt hear the pain and tears in my voice). This may sound bonkers to anyone reading this but it may be helpful to you as to how it can be done with as much dignity and as close to respectfully as possible if you were ever to consider it.