Oily looking skim on top of aquarium water

[D.K. Webster]

I have three male bettas and one female betta, all in their own tank of course. One of the males has had an oily looking skim on top of the water just about from the beginning when I set his tank up. I've made water changes, cleaning the plants and ornaments before adding water back to tank. The female had this problem but it was gone in a few days. I have been treating this same male for Pop eye but the skim was on the water way before this. This male is all the time building bubble nest so that makes me think he is happy but I'm just concerned about this skim and appreciate all the help I can get. Thank you

[AmazonJoe]

Probably protein from feeding. Could position filter to circulate the top of the water this should help.

[madagascariensis]

If you fed recently, it could be a sheen from the fats in the food.
Other than that, it is likely bacterial biofilm that indicates that you may need to watch your water quality and/or increase surface agitation.

[angelcraze2]

I agree, most likely it's a protein film you are seeing. All the bettas get fed the same foods? You can soak up the film by placing a paper towel on the water's surface. But it will come back almost daily if organics are high. When I do my water changes, I skim the surface of the water with my siphon if I notice any protein film. It won't hurt your betta unless it gets really thick and prevents oxygen exchange.