DOCs are the waste molecules skimmers are designed to remove; these are produced as byproducts from the breakdown of biological materials. This pollution arises from not only the deliberate input of foods in our tank but also from decaying organic matter (bacteria, algaes, etc). DOCs are bipolar molecules; these surfactants are attracted to air/water interfaces, i.e., bubbles. A bipolar molecule contains one or more atoms attracted to air, and one or more atoms attracted to water. A skimmer exploits this difference in the following manner: