My 2.5 gallon tank develops brown scum like spots on the walls and plants after about a month since full tank cleaning. I do 40% water changes twice a week and have a small filter/bubbler. I scrub the plants and cave about once a week to keep things clean, but these spots still develop. I think it might be bacteria colonies or something.
I have one betta and one mystery snail in the tank.
Any advice or answers as to what this is??