Red Sea REEFER DC Skimmer 900 (Excl. controller)Efficient. Quiet. Smart. The REEFER DC Skimmers include self leveling technology that prevents over skimming and full cups from spilling while continuing to aerate the water. Self Leveling. The Self leveling feature is activated whenever the wetness of the foam in the neck exceeds the user adjusted over skim threshold. The intensity of the pump is immediately reduced to prevent the over skim and then gradually increased or decreased, a bit at a time,