Do you have a toilet that always seems to be running? This is sometimes caused by a fill valve that has been set too high. Which causes the water level to be higher than the overflow tube.
All fill valves are adjustable, here are a couple common examples this one can be adjusted with a screwdriver and this one is adjusted by moving the float up or down manually because of water level that is too low, may cause a week 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