Some white film has developed on my heater holder. When you rub it, it flakes off and floats in the water. Is this something good (like good bacteria) or something else? There are no problems with the tank.

Sounds to me like a mineral deposit from your water. Has your water been evaporating at high speeds lately? The hot weather will do that. Not much to worry about. If you're itching to do something for your tank, keeping it cleaned off once a week would do the trick. I used to keep my tanks spotless inside and out in the first year. I just loved being around my tanks, rearranging, cleaning, getting my hands wet, just doing something. Ah well, the obsession wears off and the loyalty and commitment follow. Priorities just don't include attention to that much detail lately. Keeping water in peak condition is definatly still a priority for me.