[davemonkey]

Are you using DIY CO2? If so, it could be yeast making its way into the tank. Do you have any pictures?

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).

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.

-Dave