Cardinal Tetra - White/ Fungus-like Strands On Gill

This cardinal has ?always / for considerable time had a dark mark on the base of a fin and in the caudal peduncle area, accompanied intermittently by a single white spot on its tail that waxes and wanes. Nothing has been sufficiently untoward to merit (the stress of) its removal from its tankmates into a hospital tank for treatment as the isolated white spot occurrences either disappear of their own accord or after one dose of Melafix into the tank (coincidentally or otherwise).

Water-changing regime and water-quality tests are undertaken diligently weekly, without anything untoward in the readings.

However, yesterday I noticed that the right gill was white and fungused (in hindsight, I had noticed the white gill the evening before but had been otherwise distracted), and I thought treatment was necessary. The fish is the hospital/quarantine tank undergoing eSHa 2000 treatment.

Any thoughts/suggestions?  I'm thinking something like columnaris, perhaps...

https:///youtu.be/PTJF8usoUM0

The photos aren't clear at all, unfortunately, and are also showing up potentially confusing marks which are actually watermarks on the tank. However, zooming in has enabled me to see certain things on the fish that I can't see with the naked eye - and, on the basis of previous experience with other cardinals, I don't like the look of how this is progressing... (eg white spine whitening, possible fungused adipose fin). Looking at the fish straight on, the fungus on the gill actually goes right under the chin/throat, as though he has half a beard.

Incidentally, the link does work if you hover over it and copy & paste the link into a browser.

 

Hmmm - when I put it into a search engine, I get that too; however, when I put it straight into the what's-it-called (?toolbar) at the top where weblink/url addresses show, it does work.

Thanks, H. 

I think you're right about the fish being on its way out. 

I do have Maracyn-2 to hand, but having already dosed eSHa 2000 Day 1 (split into 2 half doses, 12 hours apart, to err on safe side), I think I'd realistically need at least a day to dilute that and before starting the Maracyn-2 if indeed it's appropriate treatment, otherwise the fish would be unable to cope with the sudden onslaught of chemicals.  Very sadly too, though, I don't think I have the luxury of that time...

Edited to add: Fish's buoyancy and overall colour started to go awry and, within ten minutes of noticing that, s/he died. (I started to prep a clove-oil solution but ultimately had no need to use it.)

RIP, Cardinal. 

Thanks, @Matt . Yes, so it seems re their sturdiness these days, and, likewise, I won't be purchasing them again (despite their beauty, despite theoretically they ought to do well in my water, and despite the fact that the reduction to only 3 of them has resulted in some bickering which I've never had among them before).