I don't use the in-tank chlorine tablets, and the water is potable, from the city water main. It is supposed to be safe to drink, so I don't think it has anything to do with chlorine, it has to do with rubber being rubber, it is just not suited to this application. Look at the reinforcing ribs they put on the top of the red flapper , I'd bet they knew it wasn't strong enough to begin with and designed them to help it last just past the warranty period. Great way to have you keep in contact with your plumbing professional though, when they're replacing a $10 flapper every 2 years!