Gravel is the most common substrate used for freshwater aquarium. To boost the appearance of your fish tank, decorative aquarium gravel is an ideal way. Preventing the fishes from damages it's better to keep blunt gravel for your aquarium. Size of gravel is important. 3 to 4mm is an outstanding size to allow sufficient water flow through the aquarium substrate. All gravel is not safe for fish. Some of the gravel contains toxic minerals. Always you should buy labeled gravel for your aquariums. Cultured Gravel is normal aquarium gravel with a layer of beneficial bacteria growing on the surface. Aquarium gravel is safe and providing some biological filtration. Aquarium gravels like crushed coral and reef rock (ideal for organisms that like hard water and high pH) are special. Cichlid gravels recreate the mineral content of rift lake environments. For easy cleaning gravel vacuums are available in pet shops.