You will only notice this in a relatively lightly stocked tank, but it is always happening. I only clean my cannisters about once a year. Bare bottom tanks. I use Rid X in my tanks, which you can read about in the sticky about pro-biotics. The waste, including the fiberous waste will be completely consumed by bacteria. All that is left is some black liquid that is slightly thick and sticks to the filter material and sit on the bottom along with some very fine minerals that precipitate. These are minerals that can't be consumed by bacteria. It is very fine sand. I think this is addressed in the same sticky.