We have all experienced that sound of water continuously running after you flush a toilet and the tank tries to refill. Instead of stopping, the tank just keeps refilling as water drains back into your bowl. You may also notice that the water fills the tank very slowly.
In most cases the problem can be easily fixed by just cleaning the stopper. Calcium and other minerals can often grow on the drain which breaks the seal and allows water to keep draining. You should purchase a cleaner that removes things like calcium and lime. A good one is called CLR (Calcium, Lime, & Rust Remover). However, there are many different brands.
Before you clean it turn off the water that feeds into your toilet. It can usually be done by turning or pushing/pulling a knob. Once the water is off, put some of the cleaning solution on a paper towel and rub it around the bottom of the flapper that touches the tank drain. You should also rub it on the drain surface itself. It is a good idea to wear rubber gloves when doing this, since the cleaning solution can be harmful if a lot gets on your skin. Make sure to wash your hands after using it and avoid touching your eyes.
You should next fill the tank with water again, but this time stop when the water is just above the flapper seal. You want to have water over it, but not too much. Put in some extra lime cleaner (1/4 cup is good). Allow the cleaning solution to work by sitting overnight. In the morning you can fill it back up and the toilet tank should no longer be draining once it fills. You should also notice that it fills up a lot faster because the cleaner in the water will remove stuff that was blocking water flow.
If this trick does not work, you may need to buy a replacement flapper. Just do a search online, or use Amazon and look for a toilet flapper valve. They come in different sizes, so get the one that matches your drain pipe size (3 inches and 2 inches are the most common).
You can also put some CLR in your tank once per month to help prevent the problem from happening again. This is especially important if you don’t use your toilet very often or go on vacation.