Citric acid is an excellent product for removing limescale ... so why not make your own detergent that is able to remove it and clean at the same time?
I prefer working in grams because it is more accurate than ml/tbs/oz.
HOW TO PREPARE
Pour the water into the bottle and add the citric acid. Shake the mixture and allow the citric acid to dissolve; then add the washing-up liquid.
HOW TO USE
Spray the detergent directly into the shower, bath, onto taps and the glass shower door. Allow the product to soak into the surface and dissolve the limescale: the time it takes to work will depend on how stubborn the layers of limescale and hard stains are - sometimes you will need to scrub the surface to eliminate them. Gently wipe in circular motions until the limescale dissolves. Finish by rinsing everything with warm water.
Citric acid is, of course, an "acid" and it dissolves limestone. Consequently, it is better not to use it on MARBLE (or even metals, such as COPPER or BRASS) as it could be corrosive.
Always remember to use it with your rubber gloves on when cleaning.
CREDITS TO MAMMACHIMICA