Biofilm on the surface
I have some kind of gray film on the water surface in my aquarium. This layer divides when I touch it with my finger. What is this?
such a biofilm (which, by the way, may also be greenish) occasionally occurs if there are excess organic pollutants (proteins, carbohydrates, etc.) that serve as nutrients for various micro organisms.
You can put some newspaper or paper towels on top of the surface as an immediate measure and remove it after a short while (possibly repeat several times), plus you may slightly reduce the food amount and agitate the surface well for prevention.
Dr. Bodo Schnell