Hello, My danio started showing signs of this. It was after a period of fin rot amongst my shoal, he was obviously getting low and it seemed almost like an ulcer or something. He was unable to feed as described in comments and getting very angry and fustrated and thin. I went and got frozen daphnia which is tiny little water fleas, he zoomed around with open mouth and although could not open and close I could tell he had managed to pick up some food as he immediately looked slightly less thin.
Tested by PH which was level 8 high alkaline.
I changed the tank filter to new filter as think when giving fin rot treatment it has probably killed any good bacteria on the old one. I also went to the beach and got some drift wood and stones. I washed these in the old tank water and put straight into the tank. (The drift wood helps balance alkaline water). Many pages say to boil driftwood and treat etc before putting in tank but I think sea water is quite sterile (salt kills most bacteria). My water PH improved the same day. The drift wood is leaking tannins into the water which is turning it slightly brown, I read that tannins are very good for fish.
Anyway its day 3 now since I spotted him unable to feed and mouth open and I notice he is now opening and closing mouth and managing to feed somewhat better. The daphnia was the key as without being able to feed I think he would have died and also is probably nutritious change to artificial flakes.
I am still treating every other day with fin rot treatment but tbh I don't like adding this stinking stuff to their water, feel it kills all the good stuff as well as bad so hoping the drift wood will continue to do it's job.
All fish look very healthy at the moment.
Hope this helps anyone else.
I used. Frozen Daphnia
New filter sponge
Driftwood collected from beach (hard is better) I collected a few types and tested in water taken from tank, very soft seemed to disintegrate like sand. I chose a very dark red brown piece. I was interested that drift wood is actually tree root which has purified over many years. https://fishtankadvisor.com/aquarium-driftwood/