Abstract: An aquarium tank has positioned internally on its bottom a combined aerating and purifying cartridge containing suitable filtering media. An air supply is introduced in a dual direction to the interior of the cartridge creating an even circulating flow up a centrally fitted hollow stack positioned on the upper section of the cartridge and thereby creating a suction on the interior of the cartridge. This causes water in the aquarium to flow downward to the cartridge, carrying particles of impurities with it and then the flow is circulated through the inlet holes in the cartridge and thereafter through the filter media contained within it where the impurities are removed. The cartridge is readily removed through the top of the tank without disturbing a gravel bed in which it is positioned. The cartridge is surrounded by a perforated cylindrical retaining wall held in position by the surrounding gravel bed.