there is always a slight film on the top of the water, and you should only be able to see it when you look from the bottom up, it the natural occurance. However, you can have to much by feeding live or froozen food, oil on you body when cleaning the tank. If you don't like it or it's to much, that you siphon and turn it up side down and cafefully get it close to the surface and it will suck it down and out of the tank.
I have seen a film and I think I know what you are talking about. If there is little or no circulation in the tank, it wil cause this film. Also, if you have male bettas. Their spit when the bubblenest they build disintegrates, will leave this film behind.
I would try putting a bubbler in the tank to help break up the surface.
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