Instructions
Decide if you will be planting any live plants in your aquarium. Plain gravel is not a good substrate solution for plants, as it lacks many of the basic nutrients plants require for optimal growth. If you want to consider live plants now or in the future, it may be wise to look into commercial substrates meant for plant growth.
Research the types of fish you would like to purchase and examine their feeding habits. Some species of fish, such as cories and loaches, will do much better with softer substrates, which allow them to burrow and feed without damaging their delicate barbels on rough gravel. In contrast, some larger and more aggressive feeders can accidentally swallow small pebbles or sand when they eat; for fish that are prone to this, larger stones which cannot fit in the fishes' mouths are a better solution.
Determine whether or not you are going to be using an undergravel filter in the aquarium, which will limit your substrate choices a bit. Sand will not work with this type of filter, and it may be wise to use a larger diameter, river-rock-style substrate for optimal water flow through the filter.
Choose a dark, natural-looking gravel rather than bright neon or rainbow colors. Not only will your fish feel more at home over a naturally colored surface, but darker substrates actually enhance the color of your fish, making them "pop" against the dark ground cover.
Pick a substrate that appeals to you. As long as your selection is safe for your fish and your equipment, the most important deciding factor in selecting an aquarium substrate is how it looks to you.
How to Choose Aquarium Substrate
Aquarium substrate is the start of any good tank decorating scheme, and there are literally hundreds of different brands, colors and textures to choose from. The type of aquarium substrate you use in your tank will depend on several factors, not the least of which is plain old personal preference. Choosing your substrate carefully now will prevent the headache of having to completely tear down the tank if you decide to replace it.