Substrate in Coral Reefs

"Substrate" is simply a fancy word for what you put on the bottom of your aquarium. For most reef aquarists, live sand is the substrate of choice, but crushed coral or gravel may be used with or instead of live sand, or the aquarist may opt to build a plenum beneath the substrate. Some aquarists even leave their tanks bare-bottom. Each approach has advantages and disadvantages.

  1. Live Sand

    • Live sand is sold in most places that sell saltwater fish, usually for a higher price than any other substrate. Live sand is full of beneficial bacteria which act as natural filters by turning toxic ammonia into less harmful nitrites. Live sand may also house tiny creatures, such as copepods, which provide food for your tank's inhabitants. Live sand is easy to vacuum and gives a natural appearance to your coral reef. Most experienced reef aquarists recommend a sand bed of one or two inches, though some prefer a "deep" sand bed of up to six inches.

    Crushed Coral

    • An aquarium's ability to maintain a consistent pH is known as "buffering capacity." Crushed coral increases a tank's buffering capacity and adds alkalinity, raising a tank's pH. In a reef tank, crushed coral is best utilized as a thin layer on top of live sand. Crushed coral should be vacuumed with a gravel-vac frequently to avoid buildup of waste, as debris gets caught in the craggy crushed coral.

    Gravel or Sand

    • Marine-safe gravel or sand may be used in reef aquariums, either on top of live sand or on its own. The smaller the grain, the better. More surface area provides more room for beneficial bacteria to grow. Gravel and sand (non-"live") is often the cheapest substrate available.

    Plenum

    • Some aquarists tout the use of a "plenum," or a space between the bottom of the aquarium and the substrate. The plenum is usually accomplished by egg grate or screen placed an inch or two above the bottom of the tank. The purpose of a plenum is to reduce nitrates and increase airflow in the tank. The very low oxygen level at the bottom of the substrate breaks down nitrates into nitrous oxide and nitrogen gas, which is then released into the plenum.

    Bare-Bottom

    • Bare-bottom tanks are easy to vacuum, and water flow tends to be stronger than in a tank with a sand or gravel substrate. The bare-bottom approach requires plenty of live rock to house beneficial bacteria for biological filtration. However, many aquarists feel this approach lends an unnatural appearance to a reef tank.