Red Sea Fish Pharm ARE50020 17-Inch Berlin Air Lift Protein Skimmer for Aquarium, 90-Liter Berlin air-lift is a high performance, low cost internal skimmer based on the berlin triple-pass principle The triple-pass is combined with a powerful venturi air injector, greatly enhancing the efficiency of the skimmer The air bubbles move down the chamber by the force of the water current until reaching the air return skirt located at its base The air-lift is powered by a lime wood air-stone attached to an external air pump, these compact units are easy to install and maintain Measures 17-inch height by 4-inch diameter