Add the baking soda
Sprinkle it on → it foams, that’s normal. 
It’s the cleaning action that dissolves the limescale.
Leave on for 30 to 60 minutes
For a very dirty tank: leave it to act for 2 hours.
Rub gently
Use an old toothbrush for:
the float
, the seals
, the corners
, the mechanism
