There is some reason that can happen in the beginning.
I got this oily film in my aquarium when I started it, because I did not rinsed everything before put water in the tank. Oil is often used in aquarium glass construction process... You must rinse everything before you put water.
In my case, the dechlorinator I use is making an oily film at the surface.
Has Rivercats said, it can be caused by a lack of surface agitation.
If surface agitation not solve the problem, if you have a canister filter you might want to add a surface skimmer in the input. (Low cost and effective)
A technique I do that work pretty well is to stop filter, wait 5 min, take a paper towel, and slide it from one side to the other, that should remove a part of the viscous film. Repeat if necessary.
I hope it helped
