White Fungus/Rot on Corydora

[Tap Water]

I noticed some white starting to appear on some of my bronze corydoras fins, and I cant quite figure out what it is. One cory has it worse than the rest, and it looks as if the fin is actually beginning to rot away. I've had these cory for over six months now, and I've owned cory for about four years already, and never had this problem before. They seem fine in every other aspect, it's just the white on their fins.
I'm currently treating the tank with melafix and pimafix, as instructed by my local fish-store staff/breeder and supplier of the corydora, and I hope that it works. I still, however, have no idea what the problem is, and how to avoid it again in future. Could someone please help me?

None of the other fish they're in the tank (5 cardinal, 5 rummynose, 1 female bristlenose pleco) with seem effected by this.

 

[JuiceBox52]

Could we have some pics?
It sounds most likely like fungus, I do not believe that medicine will work for that? I might though.
Do a 75% water change every day for two weeks

 

[Byron]

A photo would indeed help. Or a video if the fish keep moving.

I know how temping/reassuring it is to ask a fish store employee and then buy "x" or "y" to deal with "problem," but this often makes things worse. Corydoras are very sensitive to chemicals, and this should be the last resort and only when the issue is fairly certain. If this is fungus, there are better treatments, but we do need to see it. I nearly lost my fish using Melafix or Primafix. A good water change to get rid of as much of these substances would be advisable. It is also true that just some water changes can deal with issues, or at least help.