Leaking flappers are the #1 cause of toilet leaks. Flappers can start leaking within a day of being replaced. As mentioned, this can be due to harsh chemicals (like chlorine or bleach), or being seated improperly. I wanted to let you know about a great new product that makes detecting toilet leaks easy. The LeakAlertor electronically monitors your toilet and notifies you with visual and audible alerts when the flapper is leaking. Toilet leaks often go undetected, wasting a significant amount of water and money. LeakAlertor can help. It installs in seconds, without tools.