Just an observation to add to this...
Before I got my filter, I used a powerhead to create water movements. Had a lot of surface scum. Then I installed the filter and overnight that stuff disappeared. There was no change in surface agitation (almost none, I am using a bottom spraybar with many many holes directed 45 degrees to the front). About the same water movement, no other changes at all.
Just a thought... perhaps a bigger better filter would help... Now my surface is overgrown with Riccia, Ceradopteris, Myriophyllum and others and there is basically no surface agitation... still no more film.