If the flow of water from your Grohe shower head has lost some of its power, it's time to give it a thorough cleaning. The amount of water that can pass through the small holes is curtailed if the shower head becomes clogged with limescale or other mineral deposits. However, you don't need to head to the supermarket to search for a specific product in the cleaning aisle although a number are available to deal with this problem. Instead, open your food cupboard and pull out a bottle of white vinegar.
Remove the shower head from the water pipe with a wrench or pair of pliers to unscrew it in a counterclockwise direction. Cover the shower head with a washcloth, while loosening it, to prevent it from getting scratched. Once you've loosened it, keep turning until it comes off of the pipe. In the case of a hand shower attached to flexible tubing, remove the entire attachment by twisting it counterclockwise.
Pour white vinegar into a pot. There should be enough so that the shower head floats without touching the bottom of the pot. Add a little heat to the vinegar by warming it slightly on the stove top. While it's warming up, examine the shower head to see how much limescale has built up around the holes.
Place the shower head into the pot of warm vinegar. Make sure it is immersed then leave it to soak. Check it at regular intervals but do not remove it until the mineral buildup has disappeared. Examine the shower head and use an old toothbrush to loosen up any remaining traces of limescale. Rinse the shower head thoroughly under warm, running water.
Secure the shower head back into position with the wrench or pliers to make sure it is firmly connected to the pipe. The same applies to the connection between a hand shower and the flexible hose that supplies it with water.