Jack what I am struggling to wrap my head around is what is the difference between build up in substrate in the display tank and build up in the pads in the sump. For some reason it appears that people are less concerned with the sump having a build up in he pads than the same build up in the substrate in the display tank. To combat the build up in the display tank many forum members including myself have switched to a BB display with a sump. Now the water moving thru the pads is going to pull some of the muck and or micro organisms in the pads back into the display tank. Can't believe leaving the pads in the sump for months at a time can be good for the water quality even if the water appears crystal clear.