A common trick for people concerned about water use is to put a brick in the toilet tank. However, some Public Works departments would prefer you fill a plastic bottle with sand or rocks and put that in the tank instead. They say that bricks can disintegrate and damage plumbing.
Toilets know when to stop filling with water once the water reaches a certain level. By displacing that water with an object, you reduce the actual amount of water needed to tell the toilet it’s ready for action. It’s likely you don’t need quite the full
1.6 gallons per flush (or whatever) 10-gallon hat, so to speak, to do the job fine.
Alternatively, you can install a lo-flow toilet. — BEN POPKEN