What is a Plenum for a Reef Aquarium and What Does it Do?

In the ocean there exists a complex and amazing bio-system through which the wastes of fish and invertebrates are broken down and recycled as useful nutrients, often by bacteria and plants which are in turn eaten. In the home reef aquarium, however, even the roomiest of fish tanks don't have enough space to hold a complete ecosystem or even the thousands of gallons of saltwater per fish as found in nature.

Controlling waste nutrients such as nitrate in your reef aquarium is very important. Saltwater fish and invertebrates come from a very stable environment and they are much more sensitive to poor water quality than most freshwater species.

One common method of controlling waste products in a reef aquarium is to frequently change the water, replacing water full of waste nutrients like nitrates with newly made, clean saltwater. This is very effective - the contaminated water is diluted by the new water, lowering the levels of nitrates and other waste nutrients. Unfortunately, one cannot do this often enough to keep nitrates low enough for reef aquarium health without spending a lot of money on salt mix and expending a lot of effort mixing and changing the saltwater.

There are, however, other methods of reducing nitrates in the reef aquarium. In the ocean, de-nitrifying bacteria, along with plant life consume the nitrates produced by marine fish. Saltwater plants can be used to consume the nitrates but that generally requires a refugium or a separate, lighted reservoir for the plants to grow in. Refugia are wonderful for tank health but may not be a good option for some reefers due to the space and extra electrical demand. So this leaves de-nitrifying bacteria as the simplest solution.

Saltwater de-nitrifying bacteria only grow in oxygen-poor environments. De-nitrifying bacteria can grow naturally if the sand bed in your aquarium is deep enough but it isn't always practical to use a deep sand bed. Sand must be at least four inches deep, preferably more to harbor the de-nitrifying bacteria necessary to process nitrates in your reef aquarium. This is because the amount of saltwater that can exist in the gaps between individual pieces of sand is fairly small.

The purpose of a plenum is to create an oxygen-poor environment with plenty of water space to grow de-nitrifying bacteria. A plenum allows for the growth of de-nitrifying bacteria without the need to purchase a lot of live sand. A plenum also takes up far less room in a reef aquarium than a deep sand bed does.

A plenum is basically a riser for your sand bed. It creates a gap between the bottom of your reef aquarium and the bottom of the aquarium's sand bed where saltwater collects and circulates very, very slowly.

There are numerous styles of plenum and many methods of making plenums but all of them serve the same function. A plenum creates a space of low oxygen water beneath the sand for de-nitrifying bacteria to thrive in. Through the use of a plenum you can naturally reduce nitrates and reduce the number of water changes required to keep your fish and coral happy and healthy.

Relevant articles by this author:
How To Build a Plenum for a Saltwater Aquarium
DIY Hospital Quarantine for Saltwater Reef Aquarium
How to Acclimate Saltwater Fish & Invertebrates
How to Raise Rotifers