The most common household leak offender is a toilet. A leaking toilet can waste 200 gallons of water or more every day. Toilet leaks are typically caused by an old or worn out toilet flapper (sometimes called a valve seal).
To test for a toilet leak, place a dye tablet (which you can get for free from CVWD or purchase at a home improvement store) or 3-6 drops of food coloring in the toilet tank. If the color shows up in the bowl within 5 minutes (without flushing), the flapper should be replaced. Replacing a flapper can be a quick and easy fix. Toilet tank repair kits and parts are available at home improvement stores.