Water is a whole nuther rabbit hole.

We use one part tap water to five parts reverse osmosis water. Sometimes wife uses all reverse osmosis water but adds baking soda and magnesium.

Reverse osmosis or distilled water they say can over extract and it also can cause some erosion of the internals of the machine. The brass and copper parts. On the other hand too hard of water can cause deposits in the boilers or the machine and an off taste. There are actually hardness specifications that the experts suggest the water contain. Some people make their own water by adding certain "hardness" or buffers. Like sodium bicarbonate and magnesium but can also be others.

We mix the tap water with reverse osmosis water based on the hardness figures our rural water district put out. Not saying we get it "right". But it tastes like coffee.

Like I said, deep rabbit hole with quite a few different expert opinions. Supposedly you want the perfect balance of hardness to extract the right amount of coffee solids from the coffee beans and yet not damage the boilers in the machine.

https://www.youtube.com/watch?v=jfElZfrmlRs&pp=ygUQY29mZmVlIGFuZCB3YXRlcg%3D%3D