How big is the tank? Whats your maintainance schedule? What are you feeding and how often? What is the current stock? What are you ammonia, nitrite and nitrate levels like just before your water change? 
all Zero
I have a 22.2 gL Tank with a bristlenose and 3 tetras using an eheim 2236 so turning over more than enough,
also with live plants.
Im also do 25% water change every week without fail.
the problem is I have never got rid of it for it not to come back and I believe its a bacteria not fungul so I have been possibly treating it wrong.
Its not cottony more like a white filmy line it's hard to describe.
I have seen it hanging off and almost gone but regrows back Argh