you have protein building up. i would suggest getting a good protein skimmer. what size tank do you have? do you have a sump or will a Hang On Back skimmer be needed.
the protein skimmer will remove these proteins that are building up. that film you are starting to see is
cyano bacteria forming. there are a few causes of this, but protein build up is one of them.
when you do get a protein skimmer you will also want to get some aggitation going at the waters surface. this will break up the protein on the surface and allow the protein skimmer to remove it.
G~