(From The Daily Green website)
The old folk wisdom that says place a brick in your toilet’s water tank is partially correct: It’s an effortless way to save water, but a brick isn’t the best choice of object. Use a plastic bottle filled with water instead.
Don’t worry, you don’t have to turn hippie and “let it mellow if it’s yellow” in order to save clean water — which is an increasingly scarce resource. Most toilets will flush perfectly well with a little less H2O. The average model uses three to seven gallons per flush. A bottle in the tank will displace enough water to save half a gallon to a gallon each use, or up to about 10 gallons a day in a typical home.
All you have to do is drop a little sand or some pebbles into a bottle, fill it with water, and put it in the tank, making sure not to disturb the toilet’s working parts.
What’s wrong with a brick? They have been known to disintegrate in toilet water over time, leading to damaged plumbing. If you are a mason or otherwise determined to use a brick, wrap it in a sturdy plastic bag first. Interestingly, some utilities in Europe give away special sachets, called Save-A-Flush, which were specifically designed to displace water in toilet tanks.