Substrate refers to the material covering the bottom of the aquarium. It may be purely decorative or may serve as part of the filtration system. Several materials lend themselves to these different applications:

• Crushed coral gravel with grains averaging about 1/8 inch in diameter up to about 1/4 inch in diameter is the most commonly used substrate. Coral gravel comes bagged, and is priced by weight. As a rough guide, twenty pounds covers a square foot of aquarium bottom to a depth of about two inches. Thus, a 24 x 12-inch tank requires forty pounds of gravel.

Buying in Bulk

For installations calling for large amounts of sand or gravel, you can save a lot of money by purchasing in bulk. Sand sold for sandblasting works great and is usually sold in fifty- to one-hundred-pound bags. For still larger quantities, these materials can be as cheap as five dollars a ton, even if you purchase only a couple of hundred pounds. Industrial materials need a thorough washing before being used for the aquarium. That chore is up to you, of course.

• Natural sand comes in many colors, ranging from charcoal to eggshell, and is also available. Often, these sands are dredged from a particular area, such as Fiji. Sand, with grains generally smaller than 1 /8 inch, can be used instead of, or in addition to, gravel. You will need a little more sand than gravel to cover the same surface at the same depth, about twenty-five pounds per square foot. Avoid fine-grained beach sands, which tend to compact too densely.

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