you know, the thin filmy kind that dissolves into the water once you scrape it?
i found using a credit card to be too troubesome as 1. the area it covers isn't very big 2. the water turns greenish
so i tried out a sponge (normal sponge, no chemicals etc, soft enough not to scratch glass)
comes right off, no need to exert excessive pressure, and best of all, it sticks to the sponge so the water doesn't cloud up
you might want to try this