This fluidized bed filter is on a 300 gallon cube tank with 2 large redtail and a Gar so there are tons of solids (ugly goldfish and tilapia fillet) but they pass through sand and settle in the sump. The sand is purely a site for thin film Biofiltration which is the most efficient form. If you let the solids build up in the biomedia heterotrophs out compete the autotrophs and the efficiency drops.
If the sump is still the solids just settle out out. It's much easier to vac out a sump than to clean something like a pad that trapped the sticky carbonaceous solids(mulm/detritus). 
I vented the stack and it got rid of the bubbles. This picture is with the sharp sand without bubbles. The venting and removal of bubbles will allow the use glass beads which fluidized better than the sharp sand and have a higher surface area. 
Glass Beads, perfectly smooth and round.
If the sump is still the solids just settle out out. It's much easier to vac out a sump than to clean something like a pad that trapped the sticky carbonaceous solids(mulm/detritus). I vented the stack and it got rid of the bubbles. This picture is with the sharp sand without bubbles. The venting and removal of bubbles will allow the use glass beads which fluidized better than the sharp sand and have a higher surface area. Glass Beads, perfectly smooth and round.
