Unlike flexible flappers, the 500 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 gal per flush toilets made before 1994. 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.