Abstract: A pressure reducing control valve assembly for feeding an aeration fluid stream to a flotation tank includes a pair of overlapping discs, one having a central opening and affixed to an end of a tube, and the other imperforate and outwardly movable away from the surface of the fixed disc to define a valve opening having a predetermined land width. Optimum size of air bubbles for the objective desired is obtained by controlling the dimensions of a shear zone established by the land width within which the fluid stream is intensely turbulent for a short period of time, thereby causing air bubbles to form and then be discharged immediately into the liquid within the tank without having had time to coalesce.