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My thoughts as well. I have DIY and am careful not to let the yeast mixture get into the tube and into the water. None-the-less, I have a nice "DIY snot" build up at the outlet end of my air tube uke: that I have to clean every 2-3 weeks (although I have not noticed it elsewhere in the tank).Are you using DIY CO2? If so, it could be yeast making its way into the tank. Do you have any pictures?
If it's not this, it sounds like some kind of protein-based film. Pics and other information about the aquarium would certainly help in getting an I.D.