Abstract: Described is an aerator (10) that includes a venturi chamber (18), a pump for drawing liquid into the venturi chamber from a liquid inlet, an air inlet connected to the venturi chamber for aerating the liquid drawn through the venturi chamber, and an outlet for discharging the aerated liquid. A float chamber (26) is provided for supporting the aerator in a liquid medium, e.g. a fishing lake. Preferably, the liquid inlet and the outlet are on substantially parallel axes, wherein the liquid inlet is turnable between two positions: one position where the liquid inlet takes in liquid in an opposite direction to the outlet flow and the other position where the liquid is drawn in and discharged in the same direction. A control means responsive to sensors for determining biological oxygen demand and chemical oxygen demand is also provided for controlling the speed of the pump.