Drop by drop, litre by litre…

If you suspect that you have a leaking tap or cistern, find it and fix it. The source of a leak will not always be obvious. If you’re not sure whether your cistern is leaking, add a few drops of food colouring or water-based paint to the water in the cistern and check whether the colour appears in the toilet bowl.

Also check taps regularly to make sure they are not dripping.