REPLACEMENT FILTER MEDIA: These Cartridges are made to fit the lift tubes of Undertow and Perfecto Perfect-A-Flow filters, serving as a replacement for when it’s time to cycle out aquarium filter media. CHEMICAL FILTRATION: In addition to absorbing ammonia, nitrites, nitrates, and phosphates in a timely manner, these Activated Carbon Cartridges also remove harmful chemicals and toxins, clear up discolorations, and reduce foul odors from your tank. TOP-QUALITY: These Cartridges come together in a 2 Pack. For best results, replace cartridges every 2 to 4 weeks. They’re lightweight, disposable, and are safe for Freshwater and Saltwater aquariums. OLD SCHOOL: There has been a burst of growth, development, and ingenuity within the aquatic industry, and although we are a part of this as well, we still provide replacement media for undergravel aquarium setups. SHOP PENN-PLAX FOR ALL YOUR PET’S NEEDS: Penn-Plax is a manufacturer and distributor of top quality pet supplies, who specialize in creating excellent pet products for animals, both great and small.