Fish Health In My Tank

Thanks, @Matt.

In order to better explain it, what I'm noticing is that some of the harlies' spines seem to be more defined/demarcated vs their bodies, looking rather like the bit like the sealed part on this albeit not puckered http://www.cornishpastyassociation.co.uk/about-the-pasty/make-your-own-genuine-cornish-pasty/ with the body being the filled part underneath. [I haven't eaten a Cornish pasty for 35 years but somehow what I saw reminded me of it.] It might just be as simple as those harlies are getting plumper than the others and so their spine has a comparative demarcation line, whereas the slimmer harlies don't have a demarcation line.

I've noticed a few other odd things going on but not quite enough to cause concern yet, such as what I thought was a spine looking slightly bent/malformed when viewed from above, and possibly on the same fish a bit more of a nose-up/tail-down swimming action and a slight twitch for a split second, but just wondered about this aspect in particular while I continue to monitor that all's ok. If anyone notices anything untoward in the fish from the short-clip video, please do comment; thanks.

I see what you mean by "cornish pasty" @fcmf
At the begining of the clip, the fish that is closer to the left hand side of the tank, swims towards the left, then turns and swims back and forth a few times...as he turns the light shining above makes it look like a dark line just below the dorsal fin, then a lighter area as his body gets wider.
I don't have any harlies, and I don't think I've got any fish that are approaching the same age for comparison either.
The fish seem quite perky and active though.

I had been looking at causes of lack of appetite which might have accounted for Skinny Harley's demise and becoming increasingly concave. Fish TB is apparently a common cause of this - I sincerely hope it's not this. However, some of the other fishes' symptoms are consistent with that  eg: the harley with the wound which developed into a hole, similar to the one on the female x-ray tetra that died earlier this year after battling it for 18 months, albeit that wound is now healing; this strange curvature-type development on the harlies; the x-ray tetra's pop-eye; the increasingly bloated female x-ray tetra which died in the summer; the MDK with the missing tail portions; the MDK with the raggedy tail albeit that seems to be mending. [NB both MDKs' symptoms were present in the QT - hmmm, and the harlies' changes only developed after the MDKs entered the main tank... I wonder ] I'm not sure it accounts for the recent single white spot on the lip of one of the MDKs, though. 2 of the 4 MDKs which had an very early demise died of natural causes while 2 died of fatal accidents. The 4 new MDKs so far have had no problems, either in the QT or on being moved to the main tank a couple of days ago, although I'm now wondering about whether I was wise to move them.

The links between the 2 female tetras' demise earlier this year and the current problems may be too tenuous for fish TB to have accounted for them, and I completely appreciate that theirs and even the harlies' problems may simply be accounted for by getting older (my initial assumption) but I thought it worth getting others' views. Just to reiterate that tank water quality readings are consistently 0 ammonia and nitrite, nitrates around 20/25.

"Middle-age spread" certainly less worrying than fish TB.
We all tend to be very concerned about the health of our fish, so thinking about the worst outcomes is quite normal. 

Do you mean on top/just behind their head?

The neon green rasbora may well be a little shook up at the moment... I'd check again in another couple of hours

Yes and this is absolutely and completely normal. As harlies age they get this bump mine is very pronounced 

Rubbish photo below!

I'm worn out...

The 'traumatised' NGR has been suffering from what appears to be a catastrophic swimbladder malfunction as a consequence of being moved from the hospital tank to the main tank and thus entirely my fault (despite being done very easily and efficiently). He was lying upside down, entangled in the hornwort, possibly for some relief/rest between his nose-up, tail-down, mouth-to-the-surface interspersed with lying flat on his side and then belly-up as though gone before sudden very rapid breathing took place. Once I put the jug into the tank and rescued him, he got worse, lying completely on his side and with continued very rapid breathing.

Reluctantly, I got out the euthanasia containers. Once in the container used for The Deed, and while shaking the other container to ensure the clove oil was mixing well with the tank water, he suddenly positioned himself upright - whether there were still traces of clove oil (with vodka for the end-stage process) in it from a few days ago which have given him a temporary final reprieve, I don't know, but he's now back in the hospital tank, on the bottom, covered by a catappa leaf. I expect nature will take its course within a very short period of time, but I've decided it's best to give him a chance.
Edited to add: 5 minutes later, and he's sitting on the bottom of the hospital tank, in the correct position, wedged between an ornament and the tank wall.

RIP, Tail-less MDK/NGR      .  I feel entirely responsible for this but don't know what I would have done differently in the transfer-back-to-main-tank process - both fish were moved together, yet one thrives on return and one goes into a catastrophic demise... 

Please may I have a period of tranquillity / no fatalities or casualties in the tank..?

I think you did everything you could @fcmf
Fingers crossed for more peaceful times...

Not much of a reprieve.

One harlequin - the one which has been bobbing back and forth lately as its buoyancy has gone rather awry, with its tail lower than its body rather than the fish being completely horizontal - had an inflamed base of its right pectoral fin this morning. By mid-afternoon, I noticed the base of the pelvic fins were also inflamed. By early evening, the mouth was also affected.  See video, with apologies for debris floating past as the tank was still settling from the water change: https:#//youtu.be/P_FKm2jCVzI [remove the # to view].

I decided to do a larger-than-usual water change on the tank in an effort to mitigate this, during which I viewed all the fish from above. Currently, I have 3 fish which concern me:
* the fish in the video above
* the x-ray tetra with the pop-eye which didn't respond to treatment but is doing fine in every other way
* a harley with a very curved spine when viewed from above albeit it looks absolutely fine when viewed from the front of the tank - its spine is in a slight S shape when viewed from above, with one side of the body twisted one way and the other side twisted the other way, and this is the fish which had the injury on its side and which disappeared lately but a fleck had re-appeared in the past day or so.

I don't plan to treat the last two in the list above but am contemplating whether to treat the harley with eSHa 2000 or whether to hope the water change might be sufficient. Views welcome; thanks.

I know the harlies and x-rays are getting older but I do wonder if there's something else going on in my tank which can't be measured by standard, home-based testing kits...