It is some form of fungal disease most likely secondary to a bacterial infection.
Water changes daily - better to do smaller, more frequent wcs than huge ones sporadically. If you can do two small WCs a day, with lights off, it will reduce fish stress. Regardless of what illness it is, all fish diseases are caused by poor water quality, incorrect parameters, and stress.
Which are all one and the same really.
All treatments will have the same approach, ease the fish in and out of all conditions and perform water changes. There will be exceptions and caveats and specific meds for all the different possible things you could be dealing with, but you can always start there before worrying about anything else.
Correctly done wcs begin addressing the underlying problem causing diseases.
I like and would also be using melafix and pimafix, if fighting fit and possible isolate and treat separately.
Treat the main tank as well, half dose.
Can also use a low dose of sea salt, but do the math and use a measuring spoon, dont guess, tetras are soft water fish and wont benefit from a salt overload.
Like 2-3 teaspoons in that tank.
