One possibility is that the bacterial infection hasn't fully resolved. Does the Waterlife Myxazin mention anything about extending the treatment course? If so, you could do that. If not, you could wait for a week or two, then see if the situation improves (a latent improvement, or just to confirm that the ongoing stringy poo isn't a side-effect/result of the medication); if not, then possibly, re-treat.
What have you been feeding the fish throughout this time (just in case that may account for it)?
RO water sounds a good plan under the circumstances. Although the aquatic centres may say the fish can 'tolerate' harder, there is a difference between 'surviving' (but consequently leading a shortened lifespan) and 'thriving' (living in the conditions at which the mid-point hardness is generally the optimum level but the range cited is that which bona-fide research identifies as being able to be tolerated) - and I think you've probably gathered that and that it's better to keep the fish in the correct conditions than repeatedly having to medicate to address the effects of sub-suitable conditions.
Since posting the other day, somehow I realised I'd overlooked and omitted to include this https://www.fishkeeping.co.uk/modules/ ... esheet.php?caresheetID=72 - and it looks as though keeping WCMMs together with weather loaches is fine despite the potential size difference, and so possibly one exception to the usual rule-of-thumb of "if it fits in the fish's mouth, the fish will eat it".
All sounds great in terms of the RO unit installation (which I'm afraid I can't help with as I have no experience of these but someone else may be able to) and house renovations - I hope the fish appreciate the admirable effort you're making for them. :)
The water hardness will be fine for the WCMMs - according to the species profile https://www.seriouslyfish.com/species/tanichthys-albonubes/, they require 90-357 ppm, so your 284 ppm (CaCO3) is fine, and you might well decide to add more; they come in golden and long-finned versions as well as the more standard colour (plus some other variations). Other potential tankmates are mentioned at the end of this species profile, so you could always check those species' requirements (e.g. water hardness and tank size) against your own, but I'll have a think about some potential possiblities that would suit the temperature range of the WCMMs...
Absolutely loved your story too, and sorry I've not been able to reply before now.
Well done on doing as much research as you can (well-timed leave!) - deciphering the good .v. poor info sources on the internet can be difficult, though, but forums such as these plus the site's Articles and Caresheets sections, and Seriously Fish's species profiles are good sources - it gives up-to-date info on temperature ranges and the other requirements for each.
One factor to bear in mind, with existing and any potential additional fish you might consider, is that fish generally eat any fish that might fit in its mouth.
Make sure that you have tests for ammonia, nitrite and nitrate, at the very least - it seems that you do but the ammonia test strips are difficult to read and liquid-based tests are more accurate, so something to bear in mind either once your current batch runs out (or you might prefer to get the ammonia test in liquid format now during this crucial phase when a potential cycle or partial cycle may be underway, then use the test strips afterwards for weekly monitoring once the crucial stage has passed). For water hardness, in terms of choosing potential additional/future fish once this crucial phase has passed, your water/utility company's website will give a more accurate indicator for that than the test strips - usually in CaCO3 or German degrees hardness for your postcode - and we can then advise further on suitable potential stock.
It is very easy to get into a fluster in fishkeeping, even among the most conscientious of us (and regardless of years of experience in fishkeeping) - so I can totally empathise. The key is to keep water quality in optimum condition - ammonia and nitrite at 0, and nitrates at no more than 20 ppm above whatever the nitrate reading is for your tap water. Change ~10% daily for the next few weeks - even if water is optimum quality each day, this helps dilute other things such as stress hormones that standard test kits can't measure for - but more if any of ammonia/nitrite/nitrates is above the aforementioned levels. If water has remained at optimum levels throughout this time, it's likely that the fishtank is/was fine/cycled rather than going through a full-blown cycle, but it may take ~6 weeks of continued monitoring and water changes if water quality is continuing to fluctuate and it is going through a full-blown cycle.
As for the damaged dorsal fin, a fish can adapt to this - my goldfish (RIP) lived for a further 12 years following getting stuck in an ornament and damaging his dorsal fin which healed with a distortion. You could increase feeding to two pinches per day as the fish are likely hungry juveniles, but keep monitoring water quality as described. A good way to think of the filter is as the fishes' life-support machine so, yes - always have it on 24/7 (switch off during cleaning but don't forget to switch it back on immediately after this and tank re-filled with water).
All the best, and I think you'll make great fish "parents", having this right approach. :)
There are two ways to cycle a tank (or, more precisely the filter and its media), and each takes approx 6-8 weeks but sometimes longer: * fishless - in which the filter is prepared in advance, by adding ammonia until such a time as the filter has built up sufficient beneficial bacteria to ultimately cope with processing fish waste and fish food and the fish can be fed normal levels of food from the outset - https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm or * fish-in - which you're doing [assuming you just let the tank sit with water in it for a week with the filter switched on, in which case the water sample tested by the LFS will just be whatever your tap water is but the filter won't actually have been cycled]
Some products do exist for attempting to speed up a cycle, but the evidence for their effectiveness is still very 'hit and miss'.
Monitor ammonia, nitrite and nitrate once or twice daily for at least the next 6-8 weeks, following this advice if need be https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm . NB. The 6-in-1 test strips don't have the crucial ammonia test, so make sure you buy a liquid-based test for that as it's important that you get accurate readings for this. Doing this as diligently as you've been doing so far will maximise the chances that your fish make it through the fish-in cycle.
You should be able to increase feeding to once per day or even two smaller feeds twice per day (beneficial for juvenile fish), and increase water changes to a 10% water change daily, *provided that you keep up the water quality monitoring as described* (and take rectifying action as per link above if required).
Generally, bettas are best kept on their own (a single male on its own or a sorority of females) rather than with other fish. Sometimes, depending on the personality of the individual fish, it is possible to keep them with other fish, but it's always best to have a back-up plan if the situation goes awry - such as transferring the other fish into the larger tank (subject to compatibility with the fish in there).
The filter currently in your 60L tank will have sufficient bacteria in its filter media to support the fish in that tank (i.e. process their waste). All you need to do is fill the 200L tank with dechlorinated water (unless you use RO water, in which case ensure that it's in the same proportions as in the 60L), ensure that the temperature reaches the same as in the 60L tank, then transfer the 60L tank's filter and its media plus the fish together into the 200L. If you decide to transfer over the substrate and decor, that's fine too as some beneficial bacteria live there too albeit most of it is in the filter.
Personally, I find that: fish inhabiting the upper levels of the tank (e.g. rasboras) will quite willingly swim into a jug and thus making for an easy / stress-free transfer; fish inhabiting lower levels (e.g. tetras) are best moved via a couple of large nets, ideally guiding them into a green-coloured net which is more easily associated with safety due to being plant-coloured.
It's always best if your proposed fish are in the middle of each species' softness/hardness range. The harlequin rasbora fish, while beautiful, unfortunately require softer water than you have - your water is harder than the range they can tolerate. The honey gourami (a better choice than the dwarf gourami which is prone to the iridovirus), sterbai cory and glowlights generally prefer soft water but your water does just fall within the upper range which they can tolerate (268ppm, 15 dH, 15dH respectively .v. your 243.27 ppm and 13.63 dH). The zebra danios, platies (maculatus rather than variatus, the latter requiring harder water than you have) and guppies would be ideal for your water hardness as it's in the middle of their range.
A few points, though: * if you opt for cories - and the sterbais are gorgeous - then you'll need to consider sand rather than gravel substrate; * take note of the temperature ranges e.g.: zebra danios, platies, guppies require lower temperatures and therefore would be better together; honey gourami, sterbai cory and glowlight tetras require warmer temperatures and therefore would be better together.