Importance of Gravel

• Gravel is important to fish tanks for several reasons. First, it provides a sense of direction for fish, who may be confused by a tank that has a clear bottom. It also enhances the beauty of the tank and provides an environment that supports the growth of helpful bacteria.
  
  

Fish and Bottom Dwellers

• If fish are the only thing you're keeping in your aquarium, 2 to 2 1/4 inches should be plenty. If you want to support bottom-dwelling creatures like crabs, the gravel should be a little deeper--approximately 2 to 2 1/2 inches.
  
  

Plants

• Aquariums with live plants require two layers of gravel. The bottom layer should be composed of 1 to 2 inches of substrate. The top layer can be made up of regular aquarium gravel or sand. If you would like to keep live plants in your aquarium without the maintenance of two layers of gravel, you may want to consider putting the plants in small pots.
  
  

Under Gravel Filters

• Finally, aquariums with under-gravel filters should have about 2 to 3 inches of gravel.
  
  

Too Much Gravel

• Be careful not to add too much gravel, because it may cause the tank to become dirty quickly.