Stop a running toilet with the Fluidmaster 501 Universal Chlorine-Resistant Toilet Flapper. The 501 flapper is built to last in any water condition with its chlorine-resistant properties. The 500 Universal Flapper is best suited for use with 3.5 plus gallon per flush toilets. Only Fluidmaster has toilet flappers with built-in Microban protection. Not just simply a coating, Microban is specially built into the construction of our flappers to fight flapper breakdown and combat bacteria.