I'm assuming this tank is the twenty gallon mentioned in your signature. Goldfish are very dirty fish (which is one of the many reasons why they are not suited for betta fish or other non-pond species, however, I will not get into this), and betta fish sometimes can be sort of dirty. Goldfish make the water gross, you are overstocked, and betta fish (in tanks which are not receiving ample water movement) can secrete this sort of shiny oily sort of film. The combination of these three factors leads me to believe that the film on top of your water is from just having over all dirty fish. This does not mean they are not well cared for. Goldfish for some reason just produce more waste and dirty up water better than the average fish.
Do a water change every three days, vacuum the substrate, add an air stone (at least), and see how that goes. If at all possible, separate some of the fish. It might keep your water cleaner. If moving fish around is not possible, I'd go with just the first list I've suggested. Hope this helps.