The BOROUX Foundation™ Black filter elements are designed to remove heavy metals, but they will not remove mineral salts added by a water softener. This is because a water softener adds mineral salts to the water to make it softer. The salt used to soften water is not removed by a BOROUX Foundation™ Black filter cartridge and will actually cause the cartridge to clog. You can still use a BOROUX water filter if it’s possible to bypass the water softener. Most water softeners have a bypass valve or a way to divert water before it passes through the softener. Therefore, we strongly recommend using that water instead of running softened water through your BOROUX Foundation™ Black filter cartridges.