Abstract: An aerator for the treatment of liquid by immersion in the liquid to form in the liquid a liquid stream and introduced gas bubbles. The stream is to be produced with efficient use of power to have sufficient bubbles small enough to avoid waste by bubbles reaching the surface too quickly. The aerator has a pressure chamber (49), with a liquid inlet and gas inlets (422) positioned between the liquid inlet and a restricted outlet throat (47) associated with a chamber wall surface (44) arranged to direct an outlet stream deflected from the liquid inlet direction. The aerator can be formed without immersed moving parts by two tubes, a smaller inner one (41) expanding toward an outer larger one (42) to define the throat and pressure chamber and wall surface. The gas supply is applied through holes large enough to resist algal obstruction at a position where the gas and water enter a part of the pressure chamber of reducing cross-section for subsequent deflection out of the throat.