Pure Water Pebbles Jet Black Aquarium Gravel 2LBS (Grain size: 3.1-6.3mm). Use them in your aquariums and ponds to make the floor bottom look great. It provides living space for beneficial microbes and anchoring for plants. It also helps with the filtering process of the water in your aquarium tank or pond by increasing the surface area available for nitrifying bacteria to colonize. Recommended Uses: Fresh & Saltwater Aquariums. Ponds. Terrariums. Gardens. Potted Plants. Vases. Ash Trays. Crafts & Hobbies.