Disease ID? Desperate for help

I’ve been having a lot of trouble with Corydora habrosus that I’ve had for over a year.

1 of them recently got sick with some brown junk in their gills then passed away. Pictures of that are on my profile.

So I treated with prazi pro and expel p. Then another got the same brown junk in gills and passed away right after treatment.

Now today I have this one. Yesterday, the remaining 6 were totally healthy. Now this one has a bulging eye and infection on its face. Seems like it’s rapidly declining. If anyone can ID or tell me what medication to try next, I’ll try anything at this point.

Water params are- 75 F, 7.4 pH, 0 nitrate, ammonia, nitrite- heavily planted and established for over 2 years

Thank you so much