Just an update to report that after five years I decided to clean out my two 1350-gallon main tanks. I didn't know what I'd find. In fact they were clean as a whistle. No biofilm on the walls; not even any crud on the floor of the tank. Recall that I do use a 5-micron filter on the water that goes in, but still.
I pressure washed them with a mild bleach solution and I'm good to go for another five.