The film on your water is DOC
(dissolved organic compounds), oddly enough what makes a skimmer work, is why it is on the surface of your water. The compounds are attracted to an air/water interface (there is a really cool scientific word for it, but I can't remember it and don't feel like looking it up
). Since it is attracted to the interface, it will always be on the surface of the water as long as the tank has living organisms in it. The way to remove it, is through the use of a surface skimming overflow box for either a sump or a skimmer. If you use a sump, the overflow box will keep it churned up and either in the sump or mixed with the water where the skimmer can remove it. If you use an overflow box for your skimmer it will work, but IME, the skimmer overflow boxes are a pain. It doesn't really remove the film until the water level reaches the point that all evap is going to be noticed in your overflow box. That of course is a very small window. You can direct a PH across the surface, but it will only move it around a little, you can sop it up with some paper towels, but that is a pain as well. The best option is the use of a sump.