Within the context of the aquarium hobby, there has been a lot written about the use of sand beds as biological filters. Some of the information out there is quite good and useful while some of it is utter nonsense. Here is the very least you need to know:
1) Sand beds should be at least 4 inches deep. Given that, they usually don't function well in tanks less than 40 gallons.
2) Grain size is important. It's probably best to use sand slightly finer than table sugar. You don't need to use aragonite (calcium carbonate). Silicate sand is just fine. Even masonry sand from a hardware store should be acceptable. Whatever you choose to use, please be sure to rinse the sand before adding it to your tanks.
3) Keep the sand alive. In order for a sand bed to function as a biological filter of any kind, water needs to pass through it. This is accomplished by lots and lots of benthic (sand dwelling) organisms that squirm through and move around the sand. To keep a sand bed healthy, it's important to make sure that your sand bed is heavily populated with these little critters. Live sand bed "seeder kits" and live rock are good ways of introducing these organisms into your sand bed.