Test your toilet flapper by pouring a bit of food coloring into the tank, then check back in 15 minutes to see if the water in the bowl has changed color. If it has, then most likely your toilet flapper needs to be replaced. If this does not fix the leak, you may need to replace the entire flush valve. For more information on diagnosing a leaky toilet, watch our video “Why is My Toilet Leaking.” To learn how to change your entire flush valve, watch our video “Installing a Flush Valve.”