Abstract: An air lift diffuser assembly is made up of an air lift tube and a fixed, non-rotating bubble-emitting diffuser horizontally spaced beneath an open end of the air lift tube, the air lift tube and diffuser being releasably positioned in centered relation to a casing which is submerged beneath a body of water and is dimensioned such that the annular space between the air lift tube and casing has a greater cross-sectional area than the cross-sectional area across the tube, and an air supply pipe extends downwardly through the annular space into communication with the diffuser whereby to direct air under pressure into the diffuser which discharges the air in the form of fine bubbles upwardly through the air lift tube where the bubbles become intimately mixed with the water and dispersed in the form of dissolved oxygen over a substantial area of the body of water above the assembly.