Use less water in your toilet.

Traditional toilets, like mine, usually use more water than necessary. First, flush the toilet. After it refills, mark the water line with a crayon on the inside of the tank. To reduce the water used per flush, simply reduce the distance between the fill valve and the float. In my toilet, that means pinching the metal clasp while holding the top of the metal rod. While pinched, just slide the float down the metal rod. Don't be shy. Drop it 2 - 3 centimeters. Flush the toilet again to check the level. In the following days, if you find that you regularly need to flush twice, raise the float a bit.