An aquarium which includes a tank having a pump and an under gravel filter disposed in the tank below the pump. The under gravel filter includes a hollow bubble dispersing base plate having a perforated top surface and an overlying substrate. An air conduit is provided for introducing air into the pump. The pump is connected to the plate such as to pump blended oxygenated water and air into the interior of the plate to thereby cause oxygenated water and bubbles to exit upwardly through the perforated top surface of the plate and into and through the substrate. The pump includes a free floating magnetic impeller. A rotational torque generating unit is provided to rotate the impeller. The rotational torque generating unit includes a magnetic drive disk and a motor for rotating the magnetic drive disk. The pump and rotational torque generating unit are aligned with each other such that a magnetic field is established between the magnetic drive disk and the magnetic impeller, the magnetic field rotating upon rotation of the magnetic drive disk to thereby rotate the impeller.