One of my 9 year old comet goldfish has a growth inside it's mouth, and I'm having trouble identifying what this is and how to treat it, if it's even possible. (Photos attached below). It's stiff and strong, more like a membrane or a thick translucent layer of skin inside the mouth. It's not fluffy like I'd guess fungus would be and doesn't look like Google pictures of mouth-rot. It's spread quickly, I tried widening the hole with a cocktail stick but it didn't work.
Does this look familiar to anyone who is more confident in identifying it? My best guess is Columnaris so I'm trying Myxazin (anti-bacteria meds; Furan/Kanaplex are banned in the UK) and a daily 3 min salt bath but I'm really not sure.
The whole story:
Noticed something was wrong about 3 weeks ago when fish was spitting out food and not eating. I'd been away for a fortnight; someone came in to do the weekly water change and top up the autofeeder and I noticed when I got back. No other physical signs of anything being wrong; fish was active during the day, and attempting to eat, but not succeeding. The only symptoms were directly related to not eating (getting thinner, passing empty poo casing strings).
I read that problems with the gills could cause issues swallowing and my other fish flashed it's gills against the gravel one evening so I tried an anti-fluke medication (Interpet Anti Parasite) which had no effect.
About a week ago (not sure exactly when) I noticed something that looked like a thin layer of skin at the side of this fish's mouth but wasn't sure if it was normal or not (this fish is white so it's difficult to spot and I don't normally inspect them this closely). Two days ago it had spread and covered most of the mouth.
I'm not sure what to do; I don't know for certain what this is and how to treat it (or even if this is a secondary problem rather than a now-visible symptom of the first). Having done a lot of reading and research in the last few weeks I've realised this fish is stunted, my tank is far too small and I'm overstocked, so I've now got a pretty good idea of what caused it (probably a water quality swing) and how to improve conditions. But I think I need to do something else quickly before it seals up entirely. I have the space for a bigger tank, I just need to buy more time for this guy if possible.
Tank size: 45 litres
Tank and fish age: 9 years
Occupants: 2 comet goldfish, 1 Nerite snail (currently removed from tank because fluke medication contained copper)
Filtration: Aquael Fanfilter 1 (320 l/h)
Parameters: (readings taken daily since 11th July)
Nitrate: 10? (colours on this kit are difficult to read)
Temperature: 26 degrees (no heater; average room temperature for summer here)
Water change 25% weekly, with gravel vac on a small different area each week (though I've realised about 1/3rd of the tank wasn't being done; previously there were plants here and it never occurred to me to change the habit. I have now started on this which has chucked debris into the water column but the test kit parameters are still clear). Rinse half of filter sponges monthly in old tank water.
Recent additions: only Elodea plants from LFS
roughly every 4 weeks (a bunch was added about 3 weeks before I noticed something was wrong, I was away for a while though so not sure if that's related)
Diet: Aquarian coldwater flakes (poured in from a cup so they sink), Elodea plants. The flakes would've been floating while the auto-feeder was running.
(sorry for the textwall)