cory mold fungus

Hi, got 4 Sterbai Corydoras in the 20% off sale at pets at home.  One died within 2 days, 2 of the others have grey mold or fungus.  Put the remaining 3 into a quarantine tank.  Have tried API fungus cure and Warerlife Myxazin.  About 10 days on, 2 still moldy, the third seems immune and has no...