Penn Plax Bulk Activated Carbon for Aquarium, Long Lasting Laboratory Quality, 25 lb Bag 25 pounds of aquarium carbon bulk packed in a value pack. Carbon pellets are approximately 0.25 inch long. Laboratory-quality activated carbon is fast acting and long lasting. Designed to keep your salt or freshwater aquarium crystal clear for 4-6 weeks. Activated carbon removes harmful chemicals, toxins, odors, discolorations and other contaminants. The water will be clear and pollutant-free. Helpful tip  if you look through the side of your aquarium and cannot easily see the other side, you probably need to add some activated carbon to clear up the water. Recommended for use in cascade, and all other hang-on internal and canister filters. Thoroughly rinse all carbon fines before use.