Poly Guard GP10C System Replacement Cartridge, Scale & Rust Preventer, Up To 6 Month Supply Poly Guard Cartridge fits 10" standard whole house filter housings The cartridge will hold 1.5 pounds of crystal and treat up to 30,000 gallons of water These crystals will not dissolve completely and should be replaced in 30,000 gallon intervals or every 6 months to ensure product efficiency Easy and Economical soluiton for problem water Protects your appliances and home from hard water and iron staining