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Water Level Adjustment
Even a new toilet needs its water level adjusted within 30 days of toilet installation. That is because there are rubber diaphragms in most all ballcock (see “Toilet Parts” below) mechanisms and those diaphragms stretch like a rubber band. Also, an increase in water pressure (usually in early morning hours) can cause the water level inside the tank to rise. For example, a 15 to 20 pound pressure increase can cause the water level in your tank to rise by up to 1/2- inch, and it is not uncommon for your water pressure to rise that much during the very early morning hours.
Lower Your Water Level
To lower the water level in your tank, find the adjusting screw on the top of your fill valve. A screwdriver (or even a dime) will be all you need to turn the screw. If you want to lower the level, turn the screw clockwise on those ballcock-type mechanisms with floats hanging off the end of metal rods or plastic arms. If your fill valve has the float sliding up and down the barrel or shaft, then you will need to lower the stiff wire-like link using the "V" shaped clip. With either type, it might take you several tries to get the water to the level recommended by the manufacturer. On some toilets, the water level is shown as a line inscribed in the porcelain, others will have the line indicated on the overflow tube. If you are unable to find a water level mark then a good rule of thumb is to adjust the level to 1-inch below the top of the overflow tube and then check the level every 3 months.
Faulty Ballcock Diaphragm
In isolated cases the water will creep up on the overflow tube even though the adjustment is correct. If you find water is escaping through the overflow tube, your ballcock diaphragm is likely the culprit. If you value your time, you are probably better off to replace the entire fill valve, instead of replacing just the diaphragm. The replacement of the fill valve is relatively simple. Turn the water off at the shut-off / supply valve, remove the supply line from the fill valve, remove the flange nut on the underside of the tank that anchors the fill valve, disconnect the refill tube and pull out the old valve. It's probably a good idea to take the old valve to the hardware store and once there, check with their resident plumbing specialist regarding the best choice. There are now replacement fill valves available that are a great improvement over the old dinosaur you have with you. Install your new model following the manufacturer's instructions.
Lower Your Water Level
To lower the water level in your tank, find the adjusting screw on the top of your fill valve. A screwdriver (or even a dime) will be all you need to turn the screw. If you want to lower the level, turn the screw clockwise on those ballcock-type mechanisms with floats hanging off the end of metal rods or plastic arms. If your fill valve has the float sliding up and down the barrel or shaft, then you will need to lower the stiff wire-like link using the "V" shaped clip. With either type, it might take you several tries to get the water to the level recommended by the manufacturer. On some toilets, the water level is shown as a line inscribed in the porcelain, others will have the line indicated on the overflow tube. If you are unable to find a water level mark then a good rule of thumb is to adjust the level to 1-inch below the top of the overflow tube and then check the level every 3 months.
Faulty Ballcock Diaphragm
In isolated cases the water will creep up on the overflow tube even though the adjustment is correct. If you find water is escaping through the overflow tube, your ballcock diaphragm is likely the culprit. If you value your time, you are probably better off to replace the entire fill valve, instead of replacing just the diaphragm. The replacement of the fill valve is relatively simple. Turn the water off at the shut-off / supply valve, remove the supply line from the fill valve, remove the flange nut on the underside of the tank that anchors the fill valve, disconnect the refill tube and pull out the old valve. It's probably a good idea to take the old valve to the hardware store and once there, check with their resident plumbing specialist regarding the best choice. There are now replacement fill valves available that are a great improvement over the old dinosaur you have with you. Install your new model following the manufacturer's instructions.