Oily film on top of water with only a sponge filter

[Karen B.]

Greetings 

I usually run my aquariums with 1 HOB and 1 sponge filter.

But I have this small 5 gallons for my halfmoon betta running with only a sponge filter. 
It has sole salvinia and frogbit and duckweed as floating plants. I’ve noticed some kind of oily film forming on top of the water - I guess from the lack of water movement? Any tips on fixing it without creating too much flow for my betta? 
 

Thank you!

[Guppysnail]

Paper towels laid on the surface to suck up the oil and pull off quickly. You are correct. It is lack of surface agitation breaking it up to allow the filters to remove it. Having surface floaters along with the film can lower dissolved oxygen by preventing gas exchange. Once you remove it an extra airstone may help keep it from returning as quickly

[ScottieB]

That’s prob just a protein film…super common. Couple things things you can do… try to keep your hands out of the tank. Be careful how much organic waste is in the tank (excess food, fish poop, etc). Increase surface agitation.
To get rid of it without agitating the water you could skim it off with a cup or carefully lay a (clean) paper towel on the surface. 

Hope that helps!