Keep your aquarium's water clean and healthy with Top Fin RF-S Small Retreat Filter Cartridges. These cartridges are easily replaceable, and feature coconut shell activated carbon, which serves to absorb undesirable odors, pigments and chlorine. Advice for Use: To help maintain a healthy aquarium, change filter cartridge every 3-4 weeks, or if water flow becomes obstructed. Remove cartridge when medicating water as carbon will absorb medications. Change out 10% of the water weekly or 25% monthly. Use a water conditioner (sold separately) every time you add fresh tap water to your aquarium.