What's the EC in your tap water to begin with? Most tap water is fine to use as the EC levels are based on Ca and Mg in the water and there is no need to filter anything out. Have you looked at a water quality report for your area? Some people use filters or other systems to remove the Ca and Mg only to add it back by using calmag which is somewhat pointless. But if you have extremely hard water then you may need to do something about it.