Only Fluidmaster has toilet flappers with built-in Microban protection. The Microban protection keeps the flapper cleaner by fighting flapper breakdown due to bacteria on the flapper itself. The 501 Universal Longest Lasting Toilet Flapper is best for use in 3.5 Gal. per flush toilets made before 1994. The 501 Universal Longest Lasting Toilet Flapper is Chlorine Resistant and has a solid frame for ultimate reliability. Flappers with a solid frame do not twist and provide the best seal. Replace or fix your toilet flapper with a Fluidmaster toilet flapper.