A mould film on the water surface does not suit a well-tended tank, and it infringes gas exchange in the aquarium. A surface skimmer sucks in the water, removes the mould film and returns the filtered, clear water to the tank. This is the ideal way to remove such a film on the water surface.
Surface skimmers can be connected in line to the filter, or they can be used independently. Sometimes, it is sufficient to run independent surface skimmers at intervals. Thus the water surface is not filtered permanently, but only every few hours, depending on the thickness of the mould film.
A crystal-clear water surface without a mould film is especially attractive for the contemplator of open tanks and aquascapes, and for this reason a surface skimmer is the ideal addition to a standard filter.