Sorry, not familiar with your setup so a couple of questions. Are you running sumpless? If so, I would direct a powerhead to get some good surface agitation, you want to break up this film. If you have a sump, it sounds like there is insufficient flow into the overflow. Eitherway you definitely don't want a layer on the surface as it is cutting down the available oxygen dramatically...not good for your inhabitants. Getting the surface moving is how to get good dissolved O2.
Francis
Oops, guess I don't type fast enough! 2 posts before I was done :lol