Your choice of aquarium substrate can dictate how your little wet friends interact with their enviorment. It also provides interesting color options for your aquarium
That's right, fish like cichlids are the engineers of the aquatic world. You will see them busy on the bottom of your aquarium moving rock or swishing their tails to fan sand. They do this to create beds that they can hover over and defend like a soldier in a foxhole. When your little friends sleep, often times they will do so over a bed they have dug out in the substrate.
Your choice for the bottom of your tank can be very important. Substrate not only gives your fish an area to call their own, it also provides important surface area for those helpful bacteria mentioned on the Nitrogen Cycle page to do their work. An aquarium without substrate will have many billion fewer bacteria living in it and can degrade water quality.
There are basically three types of substrate that you should be thinking about as a novice aquarist. Each type has an advantage. The choice of color can also matter.
Choosing the type of substrate you use should outweigh the color choice. Also, color choice can matter to some types of tropical fish. Some prefer darker substrate and some prefer it to be lighter. Research your choice of tropical fish and find out what they like. A nice looking aquarium is much more interesting when your little wet friends are happy.
The substrate in your tropical fish aquarium should be one to three inches thick. Aquariums with live plants will need thicker substrate for anchoring purposes. Here is a simple formula for the amount of substrate you will want to buy.
1 pound of substrate per each gallon in your tank X number of inches in thicknes of your substrate layer = Total poundage of substrate to buy
If you end up with extra substrate, that is ok. You may need it later if you add plants or want to do some underwater construction. I like to keep an extra bag on hand just so I have it if I need it. Most substrate is fairly cheap anyway.
For more detailed information, check out Wikipedia's Aquarium Substrate page.