It's estimated that 20 percent of all toilets leak. Even if you can't hear the leak, you could be losing up to 200 gallons of water every day!
Put a few drops of food coloring in the tank. Wait 15 minutes without flushing If you see color appear in the toilet bowl, you probably have a flapper leak. To fix flapper leaks, follow these steps. More info on flappers can be found here, including alist of different flappers designed to fit your toilet.
Many toilets in leak overnight when the water pressure rises. This happens when the water level in the toilet tank rises above the overflow tube and runs down the tube constantly.
Remove your tank lid and observe the water level. If you are doing this in the middle of the night you should visibly see the water going over, or close to it. You can also sometimes see water lines in the tank where the water has risen.
If the water is higher than one inch below the overflow tube, it most likely runs at night.
Adjust it down depending on your toilet type. First, give the toilet a half-flush to empty part of the tank and make adjustments easier:
If you have a floating ball: adjust the screw on the top of the fill valve mechanism (turn clockwise) to lower the ball.
If you have a floating cylinder: adjust the "V" shaped clip on the stiff wire link (slide it down) to lower the cylinder.
Flush (full flush) again once adjusted to see where the water level rises to.
The following day, check the water level. Adjust as needed.
Excessive Flush Volume:
Not all universal flappers are universal, and a mismatched flapper can result in gallons of undetected water waste per flush. To check this, you will need to look inside the tank.
Open the tank, and then watch what happens when you flush the toilet.
The flapper should open to drain most of the water in the tank, then close after a few seconds to fill back up. When the flapper closes, there should be about 2 inches of water remaining in the tank before it starts to refill.
If your flapper stays open longer than a few seconds, then the water rushing in to refill the tank is going straight down the drain. This means you have an excessive flush volume, and a mismatched flapper.
All toilets are different, and so are flappers. Always make sure your flapper fits your toilet. Make sure you purchase the flapper that is designed for the manufacturer and model of your toilet.
Faulty Ballcock or Valve:
To repair/adjust a ballcock with a float ball: Gently pull up on the float ball rod. If the water shuts off, the float ball is the problem. It may have water in it or may be set too high. Unscrew the ball. If it has water in it, replace it.
If there's no water in the ball, the rod needs to be lowered. Use the adjustment screw on the top of the ball cock to the rod to lower the ball. The ball height should be one-half to one inch lower than the top of the overflow tube or at the water line marked in the tank. If you feel pressure and the water doesn't shut off when you pull up on the float ball rod, the ballcock is the problem.
If the ballcock looks like it is in good condition, try changing the washers in the top of the valve.
If the valve is corroded or appears broken, replace it.
To repair/adjust a ballcock with a float cup: Shut off the water supply to the toilet. Remove upper cap assembly of the ball cock by rotating 1/8 turn counterclockwise and lifting it off. Hold a container over the top of the uncapped valve to prevent splashing when the water supply is turned on. Turn the water supply valve on and off a few times to clear out debris. Replace the upper cap assembly.