A mixer nozzle assembly for mixing fluid introduced into a tank having a tank volume therein. The mixer nozzle assembly includes a stationary body and a rotatable body mounted on the stationary body for rotation about an axis in a predetermined direction. The fluid flows through the stationary body to the rotatable body and exits into the tank volume via one or more outlet apertures, thereby causing the rotatable body to rotate in the predetermined direction about the axis. The mixer nozzle assembly also includes a governor subassembly for controlling a speed of rotation of the rotatable body in the predetermined direction about the axis and a hydraulic circuit for governing a flow rate of hydraulic fluid directed to the governor subassembly, to adjust the flow rate of the hydraulic fluid for adjusting the speed of rotation of the rotatable body to the preselected speed.