Use displacement devices in toilet tanks
Older toilets use 13 or even 20 litres per flush. If you have 4 people living in your house and each person flushes 3 times a day (most people are away at school or work much of the time), a 20 litre toilet can use 240 litres of water a day— or 88,000 litres per year.
The best solution is to replace the toilet with a WaterSense® certified toilet. If that isn’t an option, you can install a displacement device. If there is space in the toilet tank, put a couple inches of sand or pebbles in a 1 or 2 litre pop or milk bottle (depending on the size of your tank— I saw a tank that easily held a 4 litre bottle). Fill the rest of the space in the bottle with water, and put the lid on the bottle. Put the bottle in the toilet tank, making sure it doesn’t interfere with the flapper or the float or any other moving parts. Test the toilet to be sure it still flushes well.