I've been setting up a planted 20g long, have had my four corydoras for at least three weeks. Today i noticed that one of the corys has a hairlike, moldy growth over his mouth. He is also lethargic, and surfaces for air much more frequently than any other fish in the tank.
After some quick googling I suspected that he may have columnaris, although I have been unable to find a good picture to make a 100% identification. Suggestions or diagnosis would be appreciated.
Today I began treating the tank with Maracyn and Maracyn-Two. Hopefully this helps.