Why does this film appear on my bettas tanks?

It's kind of gross, I clean their tanks every week, they don't have filters because filters rip their fins, I feed them pellets, could it be lack of circulation, or water pollution from the pellets! I find that my tanks with lids don't get film. I read on the internet that putting a paper towel over the film rids of it, (it works!).
Asked Feb 21, 2016

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