The CNZ Aquarium Natural River Gravel is a great way to add some depth and beauty to your aquarium, water garden, pond or terrarium. Made from natural river pebble stone, this gravel is perfect for propagating beneficial bacteria. The average size is 0.2-0.4 inches, and it’s safe for use in both freshwater and marine environments. Use two pounds of gravel per one gallon of water for optimum results.