Managuense, do you think a bio-wheel and a submerged sponge the same size would do the same amount of biological filtering? As you may or may not know, the nitrifying bacteria are far more active on the bio-wheel than any submerged media because of all the oxygen. Kind of the same idea as a fire, the more oxygen it has, the harder it works (er, burns). So even though bio-wheels may not have as much surface area as most sponges, the bacterias are far more active and work harder. The sumberged bacterias on a sponge are limited to what they can do because of the amount of oxygen in water.