Penn Plax Filt-A-Carb for Multi-Pore and Undergravel "E" Filters- 6 Total (3 Packs with 2 per pack) 6 Total Filt-A-Carb Carbon Cartridges (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 Undertow "E" Filters.