Wear rubber gloves to protect your hands.
At the proper height, the water level should be no more than 1/2 inch (12 mm) below the top of the overflow tube.
If the float ball and guide arm are operating properly, adjust the water level by using both hands to bend the arm--down to lower the water level or turning the adjustment screws on the fill valve. Flush the toilet and recheck the water level. When it is adjusted correctly, the water level will be even with the fill line on the tank or overflow tube.
If that doesn't work, you might need to service the valve. But try that first.
Piplewis
August 2006