To remove oily film from surface of the water, turn off filters. Lay a sheet of newspaper on water surface. When paper gets saturated remove. Oil will adhere to paper. May have to use a few sheets of newspaper to remove all oil. Turn filters back on.
Well I have had my tank set up for about a week or two cycling before I put fish in there. I have only had the fish in there for about a week and a half and only have done one water change
I had the same thing in my tanks till I did the same thing as you and figured out why it was happening. I think it's the proteins from the food and the fact that the surface wasn't being agitated enough.