This discussion talks about additions to Chicago water for brewing, highlights:
1) You can make good beer in many styles without treating Chicago water, espeically if you are doing extract or partial-mash brewing. Pale bitter beers are the biggest exception, see #3 below.
2) If you are mashing, you will want to add some calcium as it aids in bringing mash pH down to the proper level and is a necessary cofactor for mash amylases. So, in practice I’d do the following:
a) For pale, hoppy beers this is definitely recommended and I would use gypsum.
b) For malt-balanced beers I use calcium chloride (aids perception of roundness, fullness of malt flavor).
c) With dark beers, pH adjustment is aided by the dark grains and the natural calcium in the water will supply some of the cofactor needs, so you can get away with no Ca addition in these cases, but personally, I still add some calcium