Adopted Fish - Quarantine 'review' Time

As previously mentioned in https://forums.thinkfish.co.uk/general-fishkeeping-chat/musings-re-more-fish/msg51690/?topicseen#new , I have taken in a neighbour's fish for adoption. 

Throughout the first fortnight in the QT, the fish have adapted well, are behaving normally, shoaling nicely, actively playing by swimming through the ornaments, seem to have enjoyed the addition of a live plant, are eating well, buoyancy is good, etc.

Mr FCMF thinks they're very healthy due to the above and as they're large, and thinks they ought to be moved across into the main tank as the volume is 3x that of the QT and the downsize to the small environment from a tank of ~6x the volume might do more harm than good.

However, my trained eye, plus extra vigilance / natural protectiveness towards the existing fish in the main tank (esp after poor experiences with cardinals), notices other aspects that he doesn't and most others wouldn't.

My experience of keeping cardinals has taught me that optimum-health ones are those which maintain an extremely intense blue and red line the entire length of their bodies including gills and tail and are of a standard shape, as per the remaining two in my main tank.  I was immediately aware that the fish in my neighbour's tank were of sub-optimum quality for various reasons including this but did wonder if the light substrate, barer top of the tank, stronger lighting and larger body mass than any cardinals I've had before may have contributed to how noticeable this was.  The other issues that I'd noticed but hoped might resolve or be addressable in quarantine were the following: 
Fish 1: very large and bloated, likely a female carrying eggs but beyond a healthy size esp when viewed face-on [based on past experience, I suspect she'll succumb to dropsy in the future];
Fish 2 and 3: gill plates different - standing out rather than sitting 'flush' against the body and thus exposing the red gills - if this had been caused by ammonia, I would have expected this to have resolved following a fortnight in my ammonia-free QT, or if a problem I might have expected accompanying strange behaviour or other symptoms, so now thinking this is simply "natural variation" (eg a quick internet search reveals that some fish have short gill plates as per a discussion re koi/goldfish breeding);
Fish 3: has a small mark on its side which I think is simply a missing scale [this has not changed or developed a fungus or other problem during the fortnight in QT];
Fish 4: looks ok although a different shape to any I've had before - likely a female with a convex tummy (ie comes down into a point).

I'd appreciate your thoughts on these and esp the gill plates ie whether this indeed might just be some form of 'natural variation' or whether there is anything I can do while they are still in the QT other than ongoing monitoring for the next fortnight that all remains well otherwise - or even whether there's any merit in keeping them in the QT for another fortnight as originally intended.  NB. Have tried to take photos and video footage but difficult as the QT has no light in it and is getting reflections from the main tank and windows, and it probably won't show much up anyway.  Many thanks in advance.

Thanks, @Hampalong.

One of the cardinals in the neighbour's tank was tiny, discoloured, with a concave stomach and bent spine. I didn't take that one as I didn't think it would survive the capture and move, and don't know whether it had some genetic deformity or had fish TB. I've since learned that it died the following day.

As for these fish, I'm hereby attaching a short video clip. https:///youtu.be/4oYGV0kQHQ4 [NB to view it, just copy & paste the link straight into the url/weblink bar at the top]. These show the fish in this order:
Fish 4: likely a smaller female
Fish 3: note the gill and the small blackish mark on its side
Fish 1: large & bloated female
Fish 2: note the gill

[ Edited to add: for comparison purposes, here are the two cardinals in the main tank (with apologies for the blurriness and for the awful scratch marks down the front of the tank, thanks to an algae scraper - only really visible in photos/videos) but it illustrates what I refer to as intense colour throughout the length of their bodies: https:///youtu.be/sM6R5g2R5jE ]

Any further thoughts very welcome on the basis of the additional info/videos; many thanks.

Thanks, @Hampalong - that's very helpful indeed.

I've definitely had my cardinals for a lot longer than the neighbour has had his, and one of my two is from my first cohort (as it has two distinctive features which make it recognisable).  However, it's very possible that his could have been living in the shop for years, or a much more likely explanation that they were returned to the shop by a fishkeeper no longer able to keep them - and had previously been living in a very long tank which would probably account for their large size *.

[*I've previously mentioned that I've seen extra large x-ray tetras in a LFS which almost seemed like a different species from the size mine ever grew to.  In my 60x-30cm-footprint tank, I've never had a fish grow longer than 1 inch - but perhaps a much larger/longer tank enables that further growth.]

That's interesting. However, in my case, the tank has always had 50% water changes every week without fail.

Edited to add:

Growth rate is directly linked to the levels of the growth inhibiting pheromones that fish produce. More water changes mean less inhibitors, which means growth rate is quicker. In effect, every water change causes a growth increase.

Or might it be that the water changes dilute the pheromones that cause greater variation in size of fish within a shoal ie that the lack of pheromones present in the water prevent one fish from becoming larger/ more dominant in comparison with its shoalmates, whereas fewer or smaller water changes might have more pheromones present as not diluted and thus enable growth rate generally and the growth rate in whoever's pheromones are strongest to be larger than its shoalmates?
Thinking back to all my fish (x-rays, pygmy cories, harlequins, neon green rasbora, cardinals, espeis), most remained the same size from the outset throughout their lives relative to their peers, but each only grew to the 'expected' size. Some of the female tetras became very obese, though - and usually were the first in their shoals to succumb (ultimately to dropsy).

Ah right - ok. Thanks.

In that case, I might have thought with the smaller numbers in the main tank over the past 2.5-3 years when only 2 rather than 3 species, and with the gradual drop-off in numbers over the past year or so, that I might have seen more growth - but perhaps it needs to be much less populated than even that to show any effect.

Anyway, thanks to your reassurance that the adoptees are healthy, I think I'll move them over at the end of this week during the water change as it will be easier to do then with a lower water level (and less stressful for them), so fingers crossed that everyone integrates ok when all in the main tank despite the size difference. At least there ought to be no surprises as everyone's re-positioned themselves over the past fortnight to view one another!
 

All very interesting - thanks.

Here's a starter: https://www.practicalfishkeeping.co.uk/features/do-hormones-restrict-the-growth-of-fish/

This one’s very interesting, but strangely no mention of growth inhibiting pheromones.

https://www.practicalfishkeeping.co.uk/features/will-my-fish-grow-to-the-size-of-its-tank/

This is all very interesting indeed.

The pfk article @fcmf linked to is a super introduction to the topic and highlights the use of carbon as a potential solution to the hormone issue.

This one’s very interesting, but strangely no mention of growth inhibiting pheromones.

https://www.practicalfishkeeping.co.uk/features/will-my-fish-grow-to-the-size-of-its-tank/

Pheromones vs hormones… I’m not well versed enough to know the difference but see paragraph starting “A fish’s growth rate”

Re the seriously fish article - this shows the magnitude of the difference (in size) that can be attributed to poor husbandry - the human equivalent would be a three foot tall vs a six foot tall individual with the same genetics just living in a different environment! Add to this poor health due to “fatty organs” as per the pfk article @Hampalong posted… the human equivalent becomes even more of an interesting comparison.

Thanks for all this - I'll take a look into this in due course.

The adoptees were moved into the main tank this morning. What a "welcome"/tankwarming party they got!
The move was following a water change, and the espeis were even more frisky than usual - really didn't think that was possible! 3 were chasing around the tank, one was turning upside down under leaves, another followed suit and the two were intertwined twirling round one another while a third attempted to join in / interrupt the fun. Each plant got "christened".
The existing cardinals were pleased to have more join them.  The new very large female was more wary and exploring the tank slowly, with trepidation. The other three took a disproportionate interest in the espeis and started following them, pecking the leaves that had been "christened". Then they moved towards the snail on the filter and started pecking around her, pecking in the plant pots, and pecking snail eggs off the wood - I've never had cardinals so interested in these aspects of a tank before.

Who needs any other form of entertainment when all this activity takes place in a fishtank?!?!
 

Edited to add: the new cardinals are now obsessed with their own reflections, having not had that opportunity before; the existing cardinals have left their bachelorhood behind them on discovery of two females in the tank and are now chasing them.