About this item REPLACEMENT CARBON FILTER: Keeps aquarium water crystal clear – removes odors and discoloration. CATCHES DEBRIS: Dense, dual-sided mesh filters debris and fish waste. CLEAN WATER MADE SIMPLE: Extra-small size fits AquaCulture Aquarium Kits up to 3 gallons. CONVENIENT: Replacement filter cartridges are fully assembled and ready to use. USAGE: Change every 2 weeks or sooner if there is reduced water flow.