Outside of irrigation systems, one of the chief users of water in many households is the toilet, although the 1.6-gallon models have reduced the flush volume significantly. But what if a toilet it leaking? It doesn't matter if you have a 5-gallon or a water-efficient model. If the flapper valve is leaking (the most common trouble spot) you can waste thousands of gallons of water a month. To test if the valve is leaking, start when the tank is full and not running, take off the toilet tank lid and place a few teaspoons of food coloring into the tank (to make this a greener project, use green dye.) Without flushing, if after an hour it looks like St. Patrick’s Day in the bowl area of the toilet, you probably need to replace the flapper. There is a great Step by Step How To Video on changing the flapper (flush valve) on this site.