Conservation Tip: Did you know that (toilet) leaks can account for up to 30% of indoor household water use?
Checking for silent leaks in your toilet is easy! Put a dye tablet (free from EBMUD) or several drops of food coloring in the tank. Don't flush, wait 15 minutes to see if the dye color appears in the bowl. If it does, it's fix-it time.
You will need to replace the flapper valve and to be sure you replace the flapper with a perfect match, take the old flapper to the hardware store to compare. After replacing the flapper, do the dye test again to be sure it seals properly.
Source: Piedmont Garden Club