There is a product called Lime-Away that is used to remove lime scale and deposits from hard water.
There are a few methods you can try to remove limescale from tricky areas of a toilet bowl, but the most common is through the use of commercial cleaning products. A few examples of limescale removal products are Viakal, Lime Away and Harpic.
- Some people have some success in removing limescale from below the waterline of a toilet by adding bleach to the water. You'll need to leave this to work for a fair few hours, but it may help to remove the limescale over time.
- Another unconventional method is to pour Coca Cola into the toilet and to leave it to react for a while. You can then use a scrubbing brush to remove the limescale.
Limescale can be tricky to remove when it's below the waterline because products can't react with it too well. A combination of any of the above methods and vigorous scrubbing will likely help to remove the limescale, but it may take a few attempts.