A skimmer utilizes the natural higher surface tension of saltwater to remove disolved organics from the water.
This is how it works.
Water and air is mixed and pumped into a reaction chamber. This chamber is usually a long round tube. Within this chamber the micro air bubbles circulate and as they rise up the tube the bubbles with higher surface tension (ones with disolved organics in them) will start to stick to eachother. As they reach the top they continue to grow into larger and larger bubbles. These bubbles are then pushed up out of the main water column into a collection cup.
The resulting skum is a liquid that has a high concentration of organics and is usually the color of a dark tea.
Skimmers really do not work with freshwater because the surface tension of freshwater is much lower than that of saltwater. I have read in some books that if you wanted to use a skimmer on freshwater it would have to be very tall and have a very powerful water and airpump. The size of such a unit would make it unpractiable for all but the largest of freshwater displays.
