Aquarium gravel bed is the most commonly used material, no pollution of the water is clean before use. And the grain size of 3mm to 6mm stone a good choice for most uses. Sand is also a kind of popular substrates, and tend to be more firmly rooted plants at the aquarium. If more than two centimeters deep a layer of anaerobic decay of organic matter will occur in the pocket with no presence of oxygen to produce gas, may cause fish kills.