Use of Plenum in Refugium

Plenum is a relatively new term in aquarium keeping, meaning simply a space underneath the substrate (sand or gravel) of a refugium, separating the substrate from the bottom of the aquarium. Occasionally, use of a plenum with plenty of gravel and live sand (sand filled with beneficial bacteria and other tiny organisms) is referred to as the "Jaubert system" after Dr.Jean Jaubert, who first devised this system of maintaining marine aquariums.

  1. How a Plenum Works

    • A refugium is a tank attached to an aquarium which contains no fish. Its purpose is filtration, to add water volume to the tank, and a place for phytoplankton and copepods to grow without anything eating them. The purpose of a plenum is to reduce nitrates in a marine aquarium. The substrate of a mature refugium is chock-full of beneficial bacteria, the purpose of which is to turn ammonia from fish waste and leftover food into less harmful nitrites and, finally, nitrates. As water moves down through the substrate, nitrifying bacteria in the top level of the substrate use up most of the oxygen in the water. Nitrate, which is the final compound in the conversion, is carried down with the water towards the plenum. As the water reaches the lower levels of the substrate the nitrate is broken down into nitrous oxide and nitrogen gas by the bacteria in the very low oxygen levels of the lower substrate.

    How to Make a Plenum

    • Making a plenum in your refugium can be accomplished with some PVC piping, plastic egg grate, screening, silicone sealant, aragonite, and live sand. The plenum should leave one to two inches of space above the tank's bottom. Cut the PVC piping into equally sized pieces; the egg grate and screening will be suspended on them. Cut two pieces of egg grate to match the inner dimensions of your refugium, and cut two pieces of screen slightly larger than these.

      Put the pieces of PVC pipe in p1ace and place one piece of egg crate on them. Then position a piece of screening on the egg crate and fasten it to the sides of the refugium tank with silicone sealant. Let the sealant cure for 24 hours.

      Now add a layer of substrate--about 1 1/2 inches of aragonite, stirred to remove air pockets. Place the second piece egg grate on the aragonite, then position the second piece of screening on top of it and fasten it with silicone. Let cure for 24 hours.

      Add another 1 1/2 inch layer of aragonite, stir it to remove air pockets, and top it off with about an inch of live sand.

      It's time to fill the plenum and refugium with water. Be sure to add slowly to avoid creating air bubbles and mixing the sand into the aragonite. When operating the refugium/plenum system, little or no water flow should be directed across the surface of the substrate. Using a protein skimmer (a filter which removes organic waste) to your refugium with plenum will assure the cleanest water possible.

    Plenums in the Refugium

    • Most aquarists prefer to use a plenum in the refugium rather than their main display tanks. First, the plenum, topped with large gravel, may not be aesthetically pleasing to most viewers. Second, the structure holding up the gravel and live sand will not have to be built strong enough to hold up live rock and corals, which are in the main tank rather than the refugium.