Abstract: A filtration unit for an aquarium tank comprises a biological filter chamber which is cyclically exposed to air and water without the use of conventional syphon means. Alongside the chamber is a U-tube arrangement one branch of which communicates at its upper end with the upper region of the chamber. Compressed air is continuously admitted to the chamber, e.g. by bubbling up beneath its open bottom end. When the water in chamber drops to the level of the bend of the U-tube air in the chamber expells remaining water from the U-tube and is able to evacuate chamber, enabling it to refill with water.