The most common approach is to use a household displacement device. These devices are placed inside the tank in a place where they won't affect flushing and do just what you'd think -- displace water. Because they take up room inside the tank, the tank doesn't have to fill up with as much water each time you flush. Some typical displacement devices include plastic bottles (for example milk bottles) or bags filled with water, or bricks.
Displacement devices can reduce the amount of water a toilet uses by about 4.2 gallons (15.8 liters) per toilet per day, reducing water usage by about 13 percent, according to the Sustainability of Semi-arid Hydrology and Riparian Areas (SAHRA) research institute.
But there are cautions for using displacement devices, and they're not recommended as a long-term solution. Bricks can deteriorate and cause damage to the flushing mechanism, unless they're wrapped in plastic wrap or a small garbage bag. Plastic bottles or bags usually need to be weighted (with pebbles, for example) to make sure they're stable.
Another way to retrofit a toilet is to use a displacement dam. These are plastic dams that are wedged into the tank on both sides of the flush valve. They decrease the amount of water per flush by holding back a small amount of water. The water held back by the dams is not used, thus decreasing the amount of water used per flush. Displacement dams save about 6 gallons of water per toilet per day, according to SAHRA. One advantage they have over homemade displacement devices is that they're less likely to move around in the tank and disrupt the flushing mechanism.
Yet toilet dams and other displacement devices deteriorate over time and should not be employed as a long-term solution, according to Koeller. They definitely affect the flush performance of a toilet, which can lead to more frequent double flushing. When this happens, the expected water savings may never materialize.
Another retrofit device is the early-close flapper. An early-close flapper valve shuts off the water flow to the bowl before the toilet tank is empty. They're adjustable so that a homeowner can ideally find a level at which they can save water and still have the toilet bowl cleaned with each flush.
Still, all retrofit devices generally affect how well a toilet functions, and retrofitted toilets simply don't work as well as new low-flow and high efficiency toilets in clearing waste, according to Koeller. And since low-flow toilets with excellent performance are available for less than $100, it's hard to find reasons to choose retrofitting instead, he adds.
To learn more about low-flow toilets and other kinds of toilets, check out the links on the next page.