How to clean shower head: Why NOT to use bleach

If you can dismantle and put the parts of the showerhead in the solution separately, you’ll get excellent results.
Give the showerhead or separate parts a good scrub the following day and rinse with clean water to reveal a sparkling, limescale free showerhead.
If your shower head is really covered in limescale, you should use citric acid or a shop-bought limescale remover to get rid of the limescale.
A mixture of citric acid and water is cheaper and better for the environment than white vinegar, but both will do the job.