I use a 10 inch carbon block filter. It's usefull for filtering out any chlorine/chloramine from the town water supply. Flow through is ok, I have to change it about every six brews (on a 7 bbl system), which works ok for my brewery.
I rely on the water analysis from the town water department. I have seen breweries that had small test kits so they could test the water on brewday. This was due to the fact that their water supply came from several diff't places, and they each had diff't mineral profiles.
Head Beer Guy