*UPDATE*: If your toilet is running constantly, fix it as soon as possible. I just connected 3 months of high water bills (4x as much as a normal bill) with this issue. When your toilet is leaking, it is like a faucet running.
Difficulty Level: Easy (with proper selection and measurement)
If your toilet is constantly running, the first thing you should check is the flapper. When you push the lever that flushes your toilet, that lever pulls up a flapper that allows the water in the tank to go into the toilet bowl. This flapper can get warped (see pics of blue flapper above) over time or with use of bleach tablets in a toilet bowl – that’s actually not a good thing. When it gets warped, it can cause water from the tank to leak into the toilet – so it sounds like your toilet is always running.
I was hesitant about making this repair, even though I knew it was simple, because it has to do with plumbing. My biggest fears are flooding the place or getting shocked by electricity. I got over my fear, and went to buy a flapper.
Lessons learned: Toilets have changed in the past few years due to the new push for water conservation. So, there’s no such thing as a “universal” toilet flapper. Know what your toilet’s GPF (gallons per flush) is; most toilets are 2.0, but can range from 1.6 to 3.5+. Also, measure the chain length of your old flapper before putting in your new flapper. The chain length on replacement flappers can be adjusted based on the toilet, and it needs to be. The length of the chain on the flapper is part of the toilet’s proper flushing action.