a skimmer draws water in from the tank and mixes it with lots of tiny air bubbles disolved organic compounds (protien) stick to the surface of the bubbles and the air carries them up and into a collection cup. this removes the funk from your water before it has a chance to break down. it equates to the same action that the waves of the ocean perform. ever been to the beach and noticed the frothy smell scum thats sometimes on the surface of the water? same thing.