I don't know where it goes. In the planted tank there is alot of dead plant material in the bottom that I have to clean out. Pix later will show it. In the 150, it has a sand bottom. I never see mulm. Water is almost is colorless w/no discernible odor, like I had before in every other filter only tanks I had. I read about the waste cycle in one of these setups years ago, but I don't remember what exactly it said. I do have those Malaysian substrate snails by the 1000's in all these tanks. They are suppose to be good waste managers. I think it just decays and is washed away, if I had to speculate. Catfish and snails reduce the solids to smaller sizes.
A