I decreased the amount and frequency of food you give them, biofilm has not disappeared. After a 25% water change, biofilm reappears.
What is the solution ?
Air stone on a timer, 30 minutes a day, best when the CO2 is off. Cured.
The air stone m00se is talking about will work very well, i know many members here who do that.
or as i said a surface skimmer.
another thing that helps is when you do your WC only skim the surface.
i think i said this earlier but i have a tank that i feed maybe twice a week, and it has bio-film. i do not beileve that bio-film is a protien issue, i think it is just waste from bacteria in the tank.