Okay I had a similar problem (white cottony fuzz on a cory) yesterday and this is what I was told:
-There are two diseases which manifest as white fuzz: true fungus, and a bacterial infection called columnaris.
-True fungus is extremely rare and nowhere as lethal or fast acting as columnaris.
-Most of the time it is columnaris.
-It is really hard ot tell fungus and columnaris apart, so you are safer off to treat for columnaris first. If the fish doesn't improve with antibiotics, it means the infection is fungal, not bacterial, ruling out columnaris. So treat for fungus if it is not columnaris.
-I can't stress how important it is to treat for columnaris first. If you start with an antifungal, and the white turns out not to be a true fungus, your fish will most likely die before you realise he has columnnaris.
-I used Maracyn and Maracyn 2. It covers absolutely all the bases for both the columnaris and any secondary infections that may set in. My fish's fluff was gone overnight.
-Always finish the full course of antibiotics. If you cut the treatment off as soon as all the symptoms are gone, any remaining bacteria will become resistant to the antibiotics and then you're really screwed!