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. Unlike flexible flappers, the 501 flapper features a solid frame that doesn?t twist for the best seal every time so you never have to worry about jiggling the toilet handle again. 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.