I've had film like that on one of my tanks before. Also you get similar film on coffee pots when the coffee stains the glass pot. I've always taken a dry piece of paper towel and scrubbed it and always work, for the aquarium and coffee pot. It's just abrasive enough to basically scrape it off without scratching the glass. The paper towel will act almost like sand paper. Razor blades have never worked for me for that type of hard film on glass, it's too rigid. I use no chemicals and have never had problems removing hard film from glass with this technique. Works best when the glass is still wet, for example scraping it withing a minute or two after draining the water.
It's amazing how much gunk you can scrub off.