Do you have a toilet that always seems to be running?
This is sometimes caused by a fill valve that has been set to high which causes the water level to be higher than the overflow tube. All fill valves are adjustable.
Here are a couple of common examples
This one can be adjusted with a screwdriver.
This one is adjusted by moving the float up or down manually.
Because a water level that is too low may cause a weak or improper flush you will want to check to make sure that the water fills to the appropriate line in the tank and readjust if needed.