If you've got a little patience and some time you're willing to invest, you can take a glass tumbler and hold right-side up under the water with just the edge of the lip above the surface. Gradually lower the glass further until a tiny trickle of water is entering the glass from one side. Basically this operates as a skimmer, and removes only the very top layer of material from the water surface. It'll take a little while, but doing this a few times around the tank should remove most of the film from the surface.
