When I had an Aquaclear HOB on my old tank there was never any biofilm, this may be a function of some aquariums (regardless of food, substrate, etc.).
EDIT: My theory is the HOB filters drive the surface debris back into the water column hence no biofilm.
When my current tank was newer (first 3 to 6 months) I had a bad case, I can post pictures. I asked people on GWAPA about it, the consensus was don't worry, skim it if it bothers you but they all managed to have successful planted tanks despite the film.
It's gotten better but hasn't completely gone away, I just stopped worrying about it because it didn't kill any fish or do any damage.
You really wanna know, get a sample tested and let us all know :-)
EDIT: Here's the picture! When I put the lily pipe at the water surface there was so much bio film that it created foam - FWIW, it didn't kill anything, but annoyed me... It's gone now that the tank is 10 months old...
Last edited by sundragon; 01-14-2013 at 05:13 PM.
Reason: Additional Pictures and information