Employee was flushing out the power washer water tank with a 2 inch water hose. EE was using bleach to clean the tank. When EE pulled the hose out of the tank, the pressure of the hose pushed EE back and caused EE to become unbalanced. EE fell approx. 3 feet from the top of the toolbox to the ground. EE fractured 2 ribs and pulled a groin muscle.