I've got my pump sitting at the bottom of my sump and it works fine, however there's no sand or anything in the chamber it sits in. Is your in the tank itself, sitting on the sand? The directions with mine say to put the pump about 4" down from the top of the tank, if it's used as a HOB. I get a nice thick green skimmate running it like that. Haven't had any brown stuff yet, mainly light or dark green, depending on the dial.