In the 1950s, about the only substrate material available for aquariums was white sand
Regardless of whether a gravel or sand substrate is used, the top most layer should always be applied to a height of two to two-and-a-half inches. This allows for a sufficient area for the aforementioned beneficial bacteria to prosper, it creates a sufficient buffer between the tank bottom and bottom-dwelling species and facilitates substrate vacuuming during routine maintenance. In the case of freshwater aquarium sand, it has tendency to compact more than gravel, creating a barrier for uneaten food particle to burrow beneath the surface as is sometimes the case with gravel. In this sense, sand can actually lower aquarium maintenance times.
For aquariums that incorporate live plants, it is important to have two layers of substrate present. The bottom layer, which is the one which anchors the plants and to which their hair-like root system derives nutrients, should be labeled for that purpose. Commonly, these sorts of substrate contain nutrients needed to foment proper plant growth. The second, topmost layer should be composed of either gravel or freshwater aquarium sand. This will help keep the nutrient-rich elements of the lower substrate needed by the plants from diffusing into the water.
Using Sand for Aquarium Substrate - Oscar Fish Care
Aquarium Sand & Gravel Substrate for Saltwater & Freshwater Tanks
The bottom of the tank is one of the mostneglected parts of the aquarium, and as far as many aquarists areconcerned there really isn't much to consider. In a freshwatertank, you use gravel, and in a marine tank coral sand. If the tank hasan undergravel filter, you'll need a fair depth of the stuff, butif it's just a decorative covering to hide the floor of the tank,then you only need enough to hide the glass. So is that really all youneed to know about aquarium substrates? Definitely not! Choosing anunusual substrate is a great way to give a tank a distinctive look, andmore importantly there are many types of fish that appreciate specifictypes of substrate. Gravel and sand On the other hand, river sand cannot be used withan undergravel filter; unlike coral sand the grains are so small andcompact so readily that it cannot be separated from a gravel filter bedwithout impeding the flow of water. River sand can only be used as adecorative medium for covering the bottom of the tank, and in anunplanted tank the depth should be no greater than that which the fishcan easily move around while they're digging. The danger comes fromfood or other pieces of organic matter getting buried in the sand andallowed to decay anaerobically. Anaerobic decay is bad because itproduces toxic gases that can leak into the aquarium stressing, andpotentially killing, the fish. It is usually recommended thatsubstrates that are not part of an undergravel filter should be nodeeper than around 1 to 2 cm. Planted tanks can get away with greaterdepths, because the roots themselves will oxygenate the substratepreventing the risk of anaerobic decay, and where you are keeping bigfish, such as violet gobies or flounders, that can and do shift largequantities of sand, you can increase the depth of sand used accordingto the size of the fish in question. So with fish like catfish and spiny eels -- not tomention loaches, mormyrids, gobies, earth-eating cichlids, andfreshwater flatfish -- you really want to keep them in a tank with asofter substrate than gravel. Sand is an easy to use option, butaquarists do need to bear in mind that there are at least threedifferent types they are likely to encounter. Each has its uses, butbecause of their very different chemical properties they are not allequally suitable for any given aquarium. I like silica sand a great deal. Not only do thefish love it, but with the appropriate use of a supplement likelaterite or aquarium soil it can also make an outstanding medium foraquatic plants. But is it the perfect substrate? I think not; whileundoubtedly an versatile and inexpensive substrate it does have somemajor shortcomings. Just as with river sand, it cannot be used with anundergravel filter, and may also allow for potentially dangerousanaerobic decay. Silica sand is also very reflective, and as notedearlier, some fish fade their colours when kept over such assubstrate.