Penn Plax Cartridges FC4-Fits Undertow and Perfect-a-Flow - 6 Total Cartrides - (3 Packs with 2 per pack) 6 Total Cartrides - (3 Packs with 2 per pack) Filt-A-Carb Carbon Cartridges are made with top quality activated carbon designed to remove harmful gasses and odors, while clearing up unsightly discoloration. Undergravel Filter Cartridges are designed to keep your tank looking crystal clear. Filter Cartridges fit all Undertow and Perfect-a-Flow filters. FC4-Fits Undertow and Perfect-a-Flow