Stubborn surface biofilm?

I recently added a few new fish to my 10gal tank, and for the last week there’s been a lot of biofilm on the surface. I’ve been removing it daily with paper towels, but it keeps coming back. The tank has been cycled and doing well for 6+ months now, but I remember having this same issue for a few weeks early on. It eventually went away on its own, and I don’t think it’s harmful at all. Plants and fish/shrimp are healthy and the water parameters are stable. But it’s an eyesore and I’m curious if anyone knows what causes this? Is it related to adding the new fish?

Water stagnation is my guess. I'm guessing that tube/barrier across your filter? is blocking water flow on the surface. You could run a mini air-driven filter or air stone to help break it up elsewhere in the tank.

Tale water out from the surface, so just take out the biofilm. Increase surface agitation, like a skimmer or a stronger filter, an air stone. Problem solved. Anyway over time it settles and goes away on its own.

Ah, yep that does make sense. I put the tube there because I have floating plants that were getting ripped around by the filter/agitation…

Simple fix for floating plants and water agitation is a plant ring. Similar idea to what you have done but instead of isolating the water outflow you isolate the floating plants. Only a small area of the waters surface would become stagnant. I used to have a couple I made myself out of airline hose. They can be commonly found on Etsy and eBay