Water is a strong solvent. Therefor it is difficult to get pure H2O. Something is always disolved. Some molecules makes a big difference to the taste, others play a minor role. But it all depends on levels.
Some Calcium is good. But too much or none at all is bad.
Here in Denmark we have hard water (= high in Calcium carbonate). When you boil it in a kettle quickly scale builds up on the bottom. But apart from that it is very clean water because it is filtered through many layers of chalk in the underground.
Other places in the world surface water is used. It has less calcium carbonate, but the it has polutions and is added chlorid to kill of microbiology.
Therefor often filtered water is used for coffee brewing.
But filteret water is not just one thing. To describe how the water is different you need a lot of chemistry terminology. It is complex. And when you heat up the water it also changes.
I know many coffee people who wanted to get hold of water chemistry but have given up understanding it.
Taste different water
Best way to get the feeling of how the water impacts the taste – is to brew the same coffee with different kinds of water:
Tap water – on its own and with a little citric acid added.
Bottled water – different kinds has different specifications
Filter jug – you can get them with the classic filter (Calcium swapped with Sodium) or the newer who adds Magnesium.
Filter installation – You can ask at a coffee bar if you may fill a container with from their filter. Maybe you can find a cafe that got remineralized RO water (RO=reverse osmosis).
Best to brew it all at once in a cupping. But then you need equipment to heat up the water at the same time.
Your tap water
Normally you can get specifications on the water from your local water supplier. In most countries it is law that these data should be public.
Example from one of the water supplies for Copenhagen:
The Calcium content is 120 mg/l. This is hard water (hardness 22ºdH) but other parts of Copenhagen got as high as 165 mg/l (31ºdH). Water in Norway is much softer: 3-7 mg/l Calcium.
to be continued