Tetra - Bacterial, parasite?

[alto]

let the water go cold during a water change

This may just be coincidence - I’d expect more than a single fish to be affected ... did you happen to record the temperature drop in the tank?

(though if a fish is already health challenged, it can take a relatively minor additional stressor to begin the down trend,
eg, fish in shop seem OK, you get them home and they all show ich spots in the next couple days while fish in the shop tank still look OK (and your other fish are still as usual - in the event new fish went direct into a community tank rather than a quarantine tank))

 

[jameson_uk]

With that fish load in 180l, once you’ve done a good vacuum of the substrate and filter etc clean to sort out any possible buildup of organic waste/debris, you should be fine with a single weekly 50% water change - not to dissuade you from changing more water if that’s your preference

This was something I started when fish started to show signs of illness and I have found I have been at home more recently
I tend to give the substrate a good vacuum each water change and I have cleaned everything a couple of times. I will revert to weekly changes once my time gets limited again.

I do wonder though if low level stress is affecting the tetras, as that’s a small shoal in a big area

I think at one point there were 15 or 17 in there but the odd one has dropped off here and there leaving just the three. I am reluctant to add any more whilst this one looks like that so will give it a month or so and see if there are any more deaths.

[Quite]It does sound as if the Praziquantel has had some positive effect (I always do extra water changes when medicating, also if there were any actual “worm” parasite, then repeat the treatment after 2 weeks and possibly 6 weeks in case of eggs etc, note that time span is measured from when a previous treatment ended)[/QUOTE]
I was planning on a second treatment after two weeks but never got round to it. I will do another one later. I have also considered using Esha NDX (levamisole) to cover all bases.

This may just be coincidence - I’d expect more than a single fish to be affected ... did you happen to record the temperature drop in the tank?

I have a feeling the water going in dropped to about 18°C and the tank is normally at 25°C. This fish floated to the top during the water change so I presume the stress killed it (as you said though it probably had something else and the stress tipped it over the edge).