This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
The taste of straight water is quite subjective. Bottled
waters mostly come in two kinds, alkaline ones with massive mineral
levels just below brackish, and acidic ones with mineral levels just
above RO flatness. Very few have the intermediate hardness levels
found in most municipal waters. Moreover, the taste claims and taste
test results of bottled waters bear no obvious relation to their
chemical composition; although being as different as possible from the
local tap water may be a factor. Fizzy waters with high bicarbonates
like Perrier or San Pellegrino start out tangy acidic and become
alkaline mellow as they fizz out, so they have the advantage of
suiting any taste.
Fortunately, the effect of water on coffee is a different
story. Taste testing by various authorities over the past twenty years
has established a rough agreement that neutral pH water with 90 mg/l
hardness is optimal for coffee taste.
There's one important thing to note here. It's almost
impossible to brew coffee with neutral pH water that's too hard, since
raising it to 95C will drop out the hardness in excess of 90mg/l to
100 mg/l as scale. Asking why this is so raises some new questions.
Some water experts in Germany and Italy believe that the Langelier
Index is behind the test results. According to their theory, a
negative LI affects coffee extraction the same way acidic water would,
tasting brighter and rougher than it should. Thus, the ideal coffee
water has a neutral pH of 7, which means an alkalinity of about
50mg/l, and an LI of zero in the 90C - 95C temperature range, which
means a hardness of about 90mg/l. Notice that this approach also makes
alkalinity a factor in coffee taste.
Italian authorities, especially Segafredo, insist there is an
additional criterion, that calcium hardness is preferable to magnesium
for taste, whereas the SCAA and German authorities do not distinguish
calcium from magnesium hardness. This could be an issue with the
delivered drinking water in water coolers, which typically are RO
water with added magnesium salts.