Tank Tyrant

I’d say displacement behaviour, from the lack of a shoal...

Unfortunately the only advice I can give is to either time in QT which you’ve mentioned or buying more Harlies to re group and maybe trigger more natural behaviour.

In a few weeks I’ll be posting a review of my trip to a local fish store but interestingly while I was there I saw a group of Ember Tetras and a single Harlie which seemed to be almost a tetra in camouflage.  obviously I understand your situation where you may not want to buy anymore Harlies.

Oh good grief - I couldn't ever give a fish away.   I'm a "'til death do us part" fishkeeper.  However, she is trying my patience, and matters haven't been helped that the isolation tank option is now no longer viable for other reasons. Thankfully, the situation is not as vicious as it was yesterday.

Exactly. Furthermore, there's still the lone neon green rasbora in that tank with the curved spine round the caudal peduncle area. I had thought this might have been caused by her enjoying riding the current at a certain angle and compensatory swimming for the angle she tilts at when doing so. However, the PFK expert who replied to my e-mail several months ago did say that was unlikely and that, if there was TB* in the tank and I wasn't prepared to cull (which I absolutely couldn't do even if it was definitively TB), then to wait until all affected specimens had died and the tank was clear and symptom-free before considering any restocking.
[* No way of knowing for certain without a post-mortem if the fish with the distorted spines have died of TB or not - the elderly ones' emaciation and distorted spines may equally be accounted for by old age, although that wouldn't account for ~half the neon green rasboras' deaths of this. The lone neon green rasbora is much too active to be moved into the small QT for isolation purposes, and we simply do not have the space for a larger tank or indeed even the QT now.)]

Anyway, on a separate note, pleased to report that Wee Harley has settled down again.  She does seem to think she's one of the cardinals - certainly more related to them than the lone neon green rasbora - so I reckon her escalated diva-like behaviour was a re-establishment of the pecking order among them following the death of the large female cardinal last week.

It's just weird how attached you can get to some fish.

Absolutely. In fact, I'm semi-glad that Wee Harley hasn't been as angelic as she had been for the past 5 years. She'd always been my tank favourite as she'd been so good with the other fish and I was dreading when she would pass away. Then I developed a very strong bond with Bendy Harley during the time he lived in the isolation tank. My heart still hurts from losing him last month. I'm hoping that, by Wee Harley being not-so-perfect lately, it might make her passing less painful when the time comes!