Aquarium substrate has various functions and should be chosen according to the type of tank, i.e. freshwater, planted, fish-only, reef, etc. 
In general, the substrate provides the fish with an orientation. Glass bottoms can irritate the fish, especially if the light is being reflected from the fixture on top of the tank. Next to “orientation” and a more natural look, the substrate is breeding and colonizing grounds for the beneficial bacteria and other microscopic life forms.
Substrate for the Fish Only Aquarium
The substrate for a fish only aquarium serves the purpose of providing a surface for the beneficial bacteria to build colonies. With today’s filter technology, the size of the gravel doesn’t actually matter, but it is recommended to use a finer gravel if bottom feeders are home in the tank. The substrate should be about 2 – 2.5 inches in height, or roughly 10 pounds of substrate for approximately 150 square inches of tank surface.
Deeper layers especially with finer gravel sizes can pose a danger, given that food particles will sink into a less oxygenated area. This will over time produce hydrogen sulfite (rotten egg smell), which is highly toxic for the fish.
Types of Aquarium Substrates
Unsuitable
Sea sand is not recommended for use in aquariums because of impurities. Sea sand holds many living organisms which can die off and pollute the water. Small sea shells and coral pieces will raise the pH, it can also compact itself rather densely, which is not necessary beneficial for bottom dwellers, crabs and other sand loving fish and species.
Painted gravel or glass pebbles should not be the first choice. Glass pebbles can diffuse the light, which can irritate the fish. Painted gravel can loose its outer coating. Risk to your fish can be avoided from the start.
Common Aquarium Gravel
Probably the most used substrate for aquariums. The gravel comes in fine or more coarse varieties and consists almost entirely of quartz. It is safe to use and does not influence the water parameters at all. This gravel is suited for fish only aquariums and as top layer for planted aquariums. Regular maintenance by vacuuming the gravel is a must in order to remove uneaten food particles.
Sand
In combination with a good filtration system, sand can be the cleanest substrate of all. Sand will compact itself and food particles can not penetrate the surface. A strong filtration system will simply remove the particles 
from the surface. Having a well maintained filter (with regularly changed or rinsed filter floss) keeps a clean surface. In many cases, the filter intake tube will have to be adjusted in order to avoid the sand being pulled inside the filter. With a good filtration in place, the sand does not have to be cleaned as compared to the common aquarium gravel.
Sand as a substrate will also provide a more natural habitat for many species and bottom dwellers.
Two rather cheap sources of sand are sandblasting sand, which is more coarse than regular sand and mostly consist of quartz, and play sand, which has a finer consistency. Play sand is sterilized for the good of our children, and therefore suitable for the fish as well.
To calculate the amount of sand needed, multiply the length by width by depth of the sand you would like. The result divided by 20 will give you the approximate quantity needed in Lbs.
Aragonite
Aragonite is a calcium carbonate mineral. Aragonite sand holds a lasting reservoir of calcium carbonate, which is slowly but constantly released to buffer and to hold up the pH. Using aragonite can push the pH to about 8.2 The released calcium is a valued trace element for corals and overall functionality of the aquarium. Aragonite sand can be mixed with live sand or crushed corals. Due to the pH raising character it is suited for marine and reef set-ups only.
Crushed Corals
This substrate was a long time favorite before the use of aragonite. Sharing the same values in providing buffers to stabilize and enhance the pH, crushed corals usually push the pH up to 7.6
Most commercially available crushed corals are mixed with aragonite or similar calcareous materials such as dolomite and calcite which are similar to aragonite.
Crushed corals and the mixtures thereof are suited for marine, reef aquariums, brackish water and African chiclids in freshwater.
Vermiculate
Vermiculate is a mineral mixture of aluminum, iron and magnesium. After the mining process, vermiculite is heated to eliminate water pockets. The result is an increase in volume of 15 – 20 times it original size. The heating process where temperatures reach up to 1000 C (1800 F) makes vermiculite sterile with a high water 
holding capacity, high surface, and a neutral pH.
Vermiculite has a high cation exchange rate, which is how plants utilize the available nutrients.
Vermiculate also slowly releases valuable nutrients such as potassium and magnesium over a long period of time. It has also the ability to adsorb phosphates and to slowly transform ammonia into nitrates. Both, phosphates and nitrates can be utilized by the plants if needed.
This substrate type is ideal as a lower layer for planted aquariums. Since it has a tendency to compact itself over time, it should be mixed with laterite or common aquarium gravel to maintain the porous structure.
Laterite
Laterite is weathered clay from tropical or subtropical areas burned by the sun over thousands of years. Its porous appearance contains high levels of iron-oxide (the more reddish – brown appearance, the more iron), contributing to a high cation exchange rate. i.e. laterite attracts nutrients and holds them in “storage” until the plants can utilize them through the root system.
Laterite does not release minerals and nutrients, but can store nutrients for the plants. Laterite for aquarium use is generally baked for sterilization purposes. Cat litter is the cheapest form of laterite, but should be handled careful as most cat litters contain perfumes and clumping additives.
Laterite is an ideal lower layer or substrate for planted aquariums.
