Many theories about this skum:
Iron, organic waste, food, surface mouvement, airstone, PH...
Why so many theories? Just like with algae, many factors, different for everyone, and no definite solution.
I expierienced this with so different tanks (EI and no ferts, CO2 and non CO2, dusty and clean tanks, soft and hard water, alcaline and acidic...)
All tanks can show that film. Sometimes it goes by its self, other times you'll fight it for months.
My last terrible film was with my 60 gal Malawi tank, during a 3 months fishless cycling. Alcaline hard water, very high filtering with surface agitation, no fish, no plants, no food, no significant organic matter, no light. Nitrates near 0, no ferts, a neutral sand
That tank kept forming a so thick surface skum that forms back in 24h even more thick. I tried letting it alone for 6 weeks, it only became thicker and dustier. Later with fish in the tank, when I began water changes, the problem disappeared a while, then came back in 2 weeks. Sometimes, it forms in 24h after a waterchange, othertimes it doesn't appear before 2-4 days after the water change. Stopping food won't change this problem (as it appeared even before I add fish).
The only viable solution for such a pest is a surface skimmer
Also, nano in my signature never showed that surface film. But, since 3 months, it formed a very thick one. I kept removing it daily for 2 months without success. Now, also with a surface skimmer, I no longer have to fight it. Why it decided to begin with that film? I don't know
