The OASE CrystalSkim Aquarium Skimmers quickly and effectively free the aquarium surface of biofilm, floating plant debris and substances. It eliminates any biofilm from the water’s surface. Removing biofilm is important for proper oxygen flow in the aquarium. For aquariums from 50 to 180 gallons. The floating ring is designed to allow the CrystalSkim to skim the water's surface even if the skimmer isn't level. The CrystalSkim is a supplemental filter. It should be used in conjunction with either a power filter or a canister filter. You might want to get the CrystalSkim with the hard to find replacement sponge. Check out the three OASE CrystalSkim Videos on YouTube.