Groundhog Day - Cardinals

Sorry to hear this - it seems we are all coming to this same conclusion re cardinals… a shame

...and here we go again. This time, it's the long, healthy-appetited adoptee with the shortened gills. Colouring not quite as vibrant as the others this morning (even the other adoptees, whose colour was never as vibrant as my existing two's), panting, a little 'floaty', spine colour fading, belly whitening, ?dropsied scales, eyes protruding, not eating.

Rather than incur any delays, swift removal on noticing symptom onset into the hospital tank * with the Maracyn-2 I'd managed to acquire (still in-date) - no time to try the eSHa 2000 first. Fortunately, it was an easy task, so no undue stress induced. Hoping that, for once, swift action soon after symptom onset and before any decline has taken hold, plus no undue stress in the capturing process, might maximise the possibility of a positive outcome (unlike last time when delays were incurred in starting this treatment plus a difficult/stressful capture), but daren't lift my hopes too much. So far, so good, though; no immediate adverse experience this time - the cardinal a couple of weeks ago turned vertical in the medicated hospital tank before a swift demise.

*[Quarantine tank has the embers in it, so had to resort to the one other plastic tank. Plugged in the heater and ***flash***. Had to go and purchase a new heater from the nearest LFS before resuming my plan.]

I don't think old age is the issue, given that they were recent acquisitions for my neighbour (but can't be certain of their past prior to that - they certainly were very large).
Stress of the move is definitely a possibility - capture was difficult, move into the QT itself fortunately was swift, but the quarantine tank was a downgrade in size of tank and they lost their rummynose and silvertip swim-mates. Move from QT into my main tank was stress-free and they all *seemed* happy and well-integrated with the espeis in the past month - but one of his shoalmates did die a fortnight ago which would have been an added stress.
It's almost invariably the ones with the healthy appetites that succumb - although this one is slim/male.
Whatever the cause, I'll not be getting any more cardinals (purchased or adopted) - they've been *very* prone to these types of problems. The only other possibility, re-reading Seriously Fish's species profile, is that my water is not sufficiently acidic for them - it is naturally but the water company alter it; the water is extremely soft which is ideal for them but the PH is altered to alkaline.

Edited to add: I've seen "definite dropsied scales" (fully pineconed) in some of the cardinals in the past, and "? dropsied" where it's only really obvious with a magnifying glass and when viewed from certain angles but the slight lift of the scales alters/dims the colouring (as in this case).