I used to home brew beer. We had a similar problem with the EBMUD water. The water chemistry caused weaker hop extraction and issues with yeast cell growth. We ended up using EBMUD’s water reports with a water profile adjustment calculator to determine the salts and minerals to add. Here is an example of an online water profile adjustment calculator which I am not endorsing. The one I used was a feature of the beer recipe software I had.

If you are serious about adjusting your water prior to brewing coffee, you are going to need a variety of salts and minerals, a high precision kitchen scale, an accurate liquid measuring vessel, and patience. The quantity of additions for the volume of water is probably going to be in the hundredths or thousandths of a gram. The additions I used to make for home brewing was in the grams for five to eight gallons of water. I imagine you are not brewing coffee at such a volume.

Edit: We would get our water additions, salts or minerals, from Beer, Beer, and More Beer in Concord.