I recently took in one of my friend's male Bettas to treat it for ich (Ichthyophthirius infection). When I brought him back I noticed a film on the surface of her other Betta's tank (this one is a female). It's a small 2 gallon hexagonal tank with a mini filter, that is made for 1-3 gallons and has a very slow trickle. The same filter is powered by a pump that also runs a bubbler that she has attached by a T-valve, so both have little power. It is to my knowledge that Bettas, especially in small tanks, need little water movement due to the fact they naturally thrive in ditches and rice patties. However, what I saw made me a little concerned. Her tank has film at the top. I used her net and swiped up the film as best as I could. It wasn't sludgy, it was actually more clear, almost white as it became dry. I asked her about it and she said that only her female's tank does that. Her male is in an identical tank and he makes bubble nests all the time. She switched them from each having their own bowl to these tanks about 6 months ago and said her female's bowl had the film then, too.
I am familiar with the idea of a film being caused by a poor tank environment, algae, or not being clean enough, but her water is clear and her gravel looks clean too. I'd like to be able to recommend a fix for her if one exists, but I'm not sure if this is normal for female bettas. She has no live plants and no other fish or invertebrates in either setup, so I'm confused by there being a difference.
What could this film be? Is there a film that only females make, possibly for reproduction? Or is this a sign that maybe her tiny setup isn't doing enough? Or a mix of both?