Replacement or upgrade for 4.5-inch x 20-inch systems Dual Bone Char Carbon filters Standard 4.5-inch x 20-inch filters Replacement filters. Bone char carbon is typically used to reduce fluoride, arsenic, radioactive isotopes, iron, lead, mercury, aluminum, silver, bromine, manganese, chemicals, tastes & odors, chlorine, chloramines, herbicides, pesticides, MTBE, and THM.