Some carbon filters also filter out some ions:
"As we mentioned earlier, the Brita, and similar units, contain ion exchange resins in addition to the activated carbon elements. The purpose of these is to remove cations, in particular lead and copper but other cations, and anions, are removed as well. This was verified by qualitative tests - water passed through the test unit was virtually devoid of hardness (magnesium, calcium) and alkalinity (bicarbonate). Water treated by one of these devices for chlorine/chloramine reduction will require at least supplemental calcium and sulfate for most brewing applications and should be thought of as deionized water."
Campden tablets is a better (and cheaper) solution, imho.