If the water level is too low, it will cause the toilet to flush poorly. To adjust your toilet tank water level: If your toilet has a float arm, use the adjustment screw on top of the fill valve to raise or lower the float arm.
Keep adjusting the float until the water shuts off at the proper level. The water level in the tank is controlled by an adjustable float. A float that's set too low produces a weak flush; if it's set too high, water spills into the overflow tube and the fill valve won't shut off.