Without seeing the white stuff it's tough to say what it is. Is the white stuff fluffy looking? If it is it is probably a fungus. If the water conditions are really good ( 0ppm ammonia, 0ppm nitrite, and 20ppm or less nitrate), the fungus may go away with a few water changes over a couple of days.
If it doesn't clear up and/or starts spreading I would treat with medicine. There are alot of different types so make sure you take all your tank inhabitants tolerance (including plants and inverts), into consideration before treating and follow the instructions on the label.
Before you do treat make sure you know what it is. Google sick aquarium fish and view the images until you nail it down.
Last edited by Strider199; 09-02-2013 at 02:37 PM.
Warning; Bulldog Pleco guarding my Sons tank now..
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