Tetra 26303 Time Strip for Aquarium Filter Cartridge, 6-Pack CONVENIENT STRIPS: Place these easy-to-use strips directly into your filter system. TIMED FILTER CHANGES: Time Strips track the time to your next filter cartridge change. EASY ACTIVATION: Just press the start button to activate the timer. CLEAN WATER: Changing your water filter cartridges on the proper schedule helps keep your aquarium water clean and clear. 6-PACK: Each pack includes 6 strips and instructions.