The BOROUX Foundation™ Black filter cartridges are designed to remove heavy metals, but will not remove the 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 extracted by a BOROUX Foundation™ Black filter cartridge and will actually cause the cartridge to become blocked. You can still use a BOROUX water filter if it is 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. We therefore strongly recommend using that water instead of running softened water through your BOROUX Foundation™ Black filter cartridges.