those bio balls are where you get the foam. that is a downdraft skimmer, it uses fast moving water over the bio balls to get high foam rate. the other types of skimmers are: air driven, it uses wooden airstones to get foam,countercurrent air driven:it still uses wooden airstones but it has an added downward chamber or tube to keep the water and foam in the chamber longer. venturi:it uses a venturi valve to get its foam, air gets drawn in through a venturi valve through a proper sized pump.
aspirating skimmer: these use either a venturi attached to the back end of a pump to cause foam through the pumps impeller, or a needle wheel impeller to chop the air from the rear connected venturi. air injection skimmer: uses a very large venturi mounted at the top of the skimmer and water is injected at a very fast rate down through a tube to the bottom of the skimmer that causes a great deal of foam, this is probably the skimmer that has less maintance on it as it doesn't really need much adjustment. and last(i think last) is a beckett skimmer: it uses a special valve called a beckett valve to cause its foam. beckett valves are used on ponds to airiate them, i believe that beckett skimmers are the newest type os skimmers.
