Penn Plax Filt-a-Carb Undertow & Perfect-A-Flow Carbon Undergravel Filter Cartridge helps keep your aquarium water clean. Fits the Penn Plax Undertow and Perfect a Flow Filters. Contains quality activated carbon that removes harmful gases and odors, while clearing up unsightly discoloration. Quick easy and disposable. Helps keep tank water healthy Trouble-free maintenance Ready to use Clears up unsightly discoloration Instructions: Rinse thoroughly before placing in filter. For best results, replace every 2-4 weeks.