An aerator/mixer which includes a rotatable hollow drive shaft one or more air intake ports adjacent one end, and a propeller and an air permeable diffuser mounted adjacent to the other end; the diffuser is mounted below the propeller; and is in communication with the interior of the hollow drive shaft such that air supplied to the air intake port can exit only through the diffuser; the aerator/mixer further includes means for rotating the drive shaft and the propeller and means for supplying air at above atmospheric pressure to the or each air intake port.