To ensure that you can enjoy your fully automatic coffee machine for a long time, you should use water that is soft and free from impurities. Ideally, it should have a neutral pH value and a balanced mineral composition.
Water that is too hard and rich in minerals such as calcium or magnesium can affect the taste of the coffee by altering the extraction process and leading to a bitter or metallic taste. Calcium also leads to limescale deposits in your fully automatic coffee machine - so it is best to use water that is completely free of calcium.
On the other hand, water that is too soft can also cause problems, as it may not contain enough minerals to extract the coffee flavors properly.
In addition, water that is too soft encourages corrosion and should therefore not be used for your fully automatic coffee machine.
With perfectly matched water such as pure coffee water, you can save yourself the trouble of descaling your fully automatic coffee machine in future. This not only protects the environment, but also your wallet and the components of your machine. Regardless of the water, however, the coffee machine should be cleaned regularly according to the manufacturer's instructions (e.g. remove and clean the brew group).
For more details on the best water for coffee and espresso, we also recommend our general article on water.