Originally Posted by hypsophrys
Well... I'd hypothesize that if mulm can go somewhere, so can water w/dissolved O.
Except that the DO would be used up before the water carried it down through 4 inches of gravel. Where as Mulm wouldn't be used up, and would just accumulate. Without any plant roots penetrating down through the gravel there wouldn't be any DO delivery that way either, which was the original problem I was trying to help someone avoid.