Neon Tetra Disease (I Think)

Thanks for your kind words, both.

Of some comfort is that I managed to honour his wishes and also capture him in the net very soon post-mortem before anyone poked at him and potentially picked up the microsporidian parasite from his body if indeed this is NTD. He had been moving around the tank at odd angles and had just started floating nose down on the catappa leaf, so I removed him at that stage to check if it was the current causing the 'floating' and confirmed from the lack of movement over some time in the jug that he was dead.

What I do plan to do is to check my notes on here about the demise of the other fish since Dec in particular - I'm almost certain that their symptoms started pre- 'cardinal tetra era' (Sept 2019) and were completely unrelated to NTD (certainly its main features were not observed but the difficulty swimming and curved spine were two features although I thought more likely fish TB or simply old age), but just want to be sure. Another factor to bear in mind is that the lone neon green rasbora has had a very marked curved spine, and I can't be 100% certain that he's not carrying fish TB. I really hope there isn't something systemic in the tank but need to monitor carefully.

Updated to add: Think I have a plan of action for the future, in the event of any further incidences of this.