Take care of your aquarium the pure and natural way with Dr. Tim’s Aquatics NP-Active Pearls Aquarium Cleaner. These cleaning pearls are made with 100 percent certified biodegradable bio-polymers—a completely natural way to remove nitrate and phosphate from your aquarium. They work best in a reactor and never need to be replaced unless the reactor stops and the pearls turn black. They contain no fillers or chemicals and are safe for all types fish， corals and aquariums.