Do you have a carbon cartridge in your filter?
How many/much water changes are you doing?
Are you using a gravel vacuum?
Have you used aquarium salt?
Copper sulfate, Acriflavine, Furan, and Terramycin can all be used externally to treat it.
~Profile Picture is Kaida!~
5.5g - Female Betta: Got Julie 02/07/2014
10g - Male Betta: Got Kaida 04/24/2014