I also get this film incredibly bad. Over feeding is most definately not the cause as my betta is the only inhabitant and is fed pellets or worms one at a time and he doesnt get a single pellet or worm more than actually gets ingested.
Same with my betta, (in a 5gal with a HOB) although I must admit my bio-film isn't that heavy.
Do you have plants? If so, do you also happen to have any java ferns? Just curious...
I do have a java fern in there but if I recall the bio-film was there before the java.