Toilet flappers, the small rubber device that allows the water to flow from the toilet tank into the bowl, are often a source of annoyance and wasted water. If yours is leaking (Test by pouring a few drops of dark food color into the tank and waiting a few minutes; if the color appears in the bowl, the flapper is leaking.) replacement is simple, and will help avoid wasting both water and dollars.
Follow the manufacturer’s instructions which in most cases will involve:
Turn on water, flush, and check for proper operation. If your flapper is adjustable, as is the one shown in the photo, you can adjust the amount of water that exits the tank. This allows you to adjust for the best flush with the minimum amount of water.