Any possibility that something of this nature has occurred? https://www.practicalfishkeeping.co.uk ... -that-can-kill-your-fish/

A water change mightn't go amiss - might dilute any possible toxin undetectable by a home testkit, and certainly wouldn't do any harm (unless your water supply company mentions some sort of works going on in the area).

Can you describe what you mean by 'flashing'?
I'm not seeing anything particularly abnormal in this video, and the one you showed previously only seemed to involve the fishes' mouths making contact with various items, likely to peck off algae or other biofilm or food.

While having to treat one of my own fish for something else, I did notice on https://www.waterlife.co.uk/tropical that Waterlife Paragon *might* be ok to try with invertebrates - but you'd be best to contact Waterlife directly to ask about this.

I don't think you should be treating, though, unless you know something is awry. If you could get a video of the actual behaviour you call 'flashing'', it would make it much easier for us to help accordingly.

Sorry to read about that. It may be that they had it pre-purchase (eg others in the tank had it) or it may be that the collective stress (of their journey to the shop, then from shop to home, and then new environment in possibly slightly different conditions than in the shop) was all too much for them and they succumbed to it.

More importantly, though, if you didn't quarantine them for a month or so in a quarantine tank and instead put them straight into the well-established 200L tank, those fish in the main tank are now at substantial risk of succumbing too. It would be worth treating the main tank with eSHa Exit or Waterlife Protozin.

Welcome.
Apologies for the delay - been unable to reply before now.
The fish may very well be pregnant - even ten seconds in a tank with a male can result in enough sperm stored to produce offspring for the next 6 months!
It looks as though the fish in the background may be male and the one in the foreground female - but check picture at https://www.fishkeeping.co.uk/modules/ ... wtopic.php?post_id=352044 for how to differentiate males from females.
Alternatively, if they've eaten a lot at once, they may look similar/bloated - feeding something like peas regularly can help with that (shell off, chopped into tiny morsels).
https://www.seriouslyfish.com/species/xiphophorus-variatus/ provides details of how to care for these fish.

I never think it's wise to treat unless the fishes are definitely ill (and the platy has just recently undergone medication).
Do you know if that's the BN's normal colouring (there's always variation between fish) or if this has changed? [Even that might be difficult to tell, if she's a baby - it may be what becomes natural for her.]
I would continue quarantining both the BN and the platy for a full month - longer if you're not sure what's happening. If the fish don't eat, or further symptoms develop, then it might be wise to treat with a broad-spectrum medication or with something more specific if it seems that's required.

This is likely the only reply I'm going to be able to post today, so I'll be as comprehensive as possible:
* the video you showed us did not look like flashing - it looked like fish trying to eat algae from specific locations around the stringray filter;
* if the fish are doing similar behaviour off the gravel, it's very likely they are trying to eat algae or assassin snail eggs laid on it or possibly food debris;
* the water quality seems fine and the fish don't seem to be ill or doing anything unusual besides this behaviour, so, if it's irking you, then you'll need to consider alternatives such as a separate tank for the snails to avoid snail eggs, ensuring no algae build-up on the filter and elsewhere, and having a substrate that doesn't attract algae and is easier to remove food debris from, to avoid the fish trying to eat these;
* as the fish don't seem to be ill, it would do more harm than good to medicate, but the only two products I'm aware of that are safe to use with invertebrates (snails or shrimp) are API Melafix and API Pimafix;
* if you think the behaviour is different to this, then please do capture examples of it on videos to post on here, and we'll try to advise further - but, if medication is required for the tank, you'll probably have to create a new tank for the snails from then onwards.

Hmmm. There's nothing untoward leaping out at me (other than your last boss - glad he's in the past if that's his view of goldfish!). However, test strips tend to be very unreliable, which is why liquid-based ones are usually recommended. [Personally, I found the API ammonia test strips very difficult to read too, and the test strips particularly unreliable for PH/GH/KH.] I would get liquid-based ones for ammonia, nitrite and nitrate at least.

In the interim, I'd do the following to see if that helps:
* do more frequent and larger water changes such as 2-3 x 30% water changes per week;
* pre-soak the food in a small container of tank water before pouring it in;
* feed smaller portions more frequently;
* a one-off dose of aquarium salt (which will naturally dilute with subsequent water changes);
* check out https://injaf.org/aquarium-fish/the-go ... bladderbuoyancy-problems/ to see if there's anything else you haven't tried.

I don't think the water hardness is of any concern.
What is much more important, though, are:
* the *exact* readings for ammonia/ammonium, nitrite and nitrates, plus details of the test kit you're using (as there's some variation in them and results are best interpreted in that context);
* tap water readings for the above;
* how many fish there are altogether, and size (dimensions and volume) of tank, and water maintenance regime;
* anything else that is added besides water dechlorinator?
These are crucial for cases such as these and, once we have these details, we can advise better.

I'm not convinced that's flashing - to me, it looks like the fish noticing something potentially edible and using a technique to lift it up (eg the algae on top of your filter). I've seen my own fish do that if there is ever algae on a plant, a morsel of food on the substrate or a nerite snail egg on the glass. Try removing that bit of algae from the top of the filter and around the circular 'line' below that, and see if that stops them doing it there. If that works, and the behaviour continues to irk you, then you might then want to consider a substrate easier to clean and less prone to attracting algae or food debris wedged between the pieces eg sand.

Ah - that second photo is very helpful for an added perspective. What I thought was the patterning of the goldfish (white colouring) looks fungal to me. I think an anti-fungal medication is probably best.

Updated to add: Waterlife is a good brand and I see they do a pond treatment - Medizin P - that might very well help: https://www.waterlife.co.uk/pond