Petco Nautical Star Aquarium GravelPetco Aquarium Gravel is ideal for freshwater and safe for marine aquariums. This is a high quality, non-toxic gravel that helps aid in the propagation of beneficial bacteria. It will not affect the water"s chemistry nor harm any aquatic life (fish, invertebrates or plants). Create a healthy environment for your fish by combining gravel, plants and decor which provide cover and hiding places.