There is actually great debate surrounding sand and substrate choices-- live sand included. You can find a wonderful article discussing the merits of various sand bed types here: Great Substrate Debate.
So… What is live sand and what does it do? “Live sand” is a loosely used term to describe fine, usually aragonite based sand that has been previously colonized by bacteria. In many instances, if sand is purchased from a reputable dealer or is collected from a well established tank, live sand will have many worms, shrimp, crabs, starfish, and a variety of other micro and macro-fauna. I recently tore down a tank and discovered thousands of beautiful sand dwelling feather duster worms. It was a veritable forest!
Live sand and sand beds have a variety of functions in the reef aquarium. Live sand works much like live rock in terms of biological filtration, but in different ways. For instance, the most basic function of live sand is to provide the necessary bacteria and surface area to break down wastes that other forms of filtration may miss. The surface of the sand bed supplements aerobic biological filtration, facilitating the breakdown of ammonia to nitrite, and nitrite to nitrate. Sand beds that are adequately deep have been known to develop anerobic pockets that allow bacteria to break down stubborn nitrates into inert nitrogen gas. These anaerobic pockets have a slightly lower pH than the rest of the water column. As a result, it is believed that in those pockets, small amounts of the aragonite sand is dissolved, providing a minor source of calcium, alkalinity, and trace element supplementation to the tank.
Live sand, like live rock, hosts a wide variety fauna and micro-fauna. This contributes to the biodiversity within the reef tank and provides another way to process waste. Bristle worms, spaghetti worms and brittle stars are common hitchhikers that help to consume dead and decaying matter. And many small crustaceans and hitchhiking gastropods contribute to consuming film and filamentous algae from the sand bed and rocks. An active live sand bed is continually turned over by the multitude of small animals, working through the detritus and carrion that accumulates on the sand surface.
In addition to filtration, an active live sand bed is usually home to a variety of copepods, isopods, decapods and so on. These tiny crustaceans become a natural food source for planktavores and filter feeders in the aquarium.
So…Do you need to buy live sand? The short answer is no. If you are setting up a reef tank for the first time and are making your initial purchases, buying high quality, uncured live rock, such as Tonga Lalo, or Fiji rock, can help contribute a great deal to the biodiversity of dead/dry sand. Dry sand is soon colonized by a variety of animals hitchhiking from the live rock turning it relatively “live.”
If you have an already established tank, and wish to increase the biological diversity in your sand bed, a cup or so of sand from an established reef tank can help seed your existing, sterile sand. Organisms present from the established reef tank will proliferate and colonize the sand bed; your entire sand bed is soon “live."
Why purchase live sand? Live sand, like live rock, helps to provide an initial biological “kick start.” High quality live sand comes teeming with microscopic life. If starting with all dry sand, you may never have the initial seed to develop the kind of bio-activity and diversity found in a mature live sand bed. It is an important consideration when setting up your new tank. Like high quality live rock, high quality live sand is priceless.
So how do you purchase live sand? Live sand comes in a variety of forms. A lot of live sand is sold in prepackaged, superoxygenated, wet bags. Carib-Sea’s Arag-Alive product is a wonderful innovation in live sand distribution. Each prepackaged bag comes teeming with microbial life, despite being sealed in, and comes with an expiration date to assure you that your stock is fresh and viable. Though teeming with microbial life, prepackaged, wet livesand is generally devoid of micro and macrofauna like pods and worms.
Other live sand is sold in moist boxes. This live sand is collected directly from the ocean on a reef-side reserve of sand. Like live rock, it is directly imported from Fiji. As this sand is collected on the reef, it comes teeming with micro and macrofauna. But, like live rock, live sand that is shipped moist and not completely submersed in saltwater experiences some die off and must be added to a new tank or cured in the same fashion as live rock. If possible, this is one of the best routes of adding live sand to the aquarium, as it adds microfauna that may not be present in live rock, contributing further to the biodiversity of the sand bed.
So when considering the substrate in your next tank, consider live sand and all of its merits. It is a wonderful investment that helps ensure the health and diversity of your reef aquarium. A sand bed teeming with life is both beneficial and beautiful. Sometimes the smallest organisms make the largest impact on your reef aquarium.