Oily film on the water's surface is usually from oily fish foods. Fresh carbon and agitation are ways to break it up. You can also "wipe" it with a paper towel. Or skim it with a cup, or siphon the surface when you water change. Protein skimmers are not as effective on freshwater tanks as saltwater tank, but they can be used for this purpose. A little oil on the surface won't hurt anything, but if it continues to build up the oxygen exchange between the water and air is reduced. Oxygen exchange is increased by increasing agitation also.
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