In almost any aquarium a film of including microorganisms dust oils and food fats forms over time the mould film with the eheim skim350 surface skimmer you can efficiently eliminate this mould film and have a glass clear water surface and high oxygenation the eheim skim350 although designed for constant use is also suitable for short term use if using alternately in several aquariums please make sure that algae spores or diseases are not transmitted from one to the other the bacteria