How to Build a Plenum for a Saltwater Aquarium

Nitrates build up in a cycled aquarium as a result of bacteria processing fish wastes into nitrites and then nitrates. Nitrates are a harmful waste product if their levels get too high. Fortunately, there are also bacteria which eat nitrates. Unfortunately, those bacteria can't thrive in the same water conditions your fish and invertebrates need to survive. Nitrate eating bacteria require very low oxygen concentrations to live.

A plenum is basically a riser that is placed under the sand or crushed coral of the saltwater aquarium to make an "empty" gap between the bottom of the tank and the bottom of the sand or crushed coral. A plenum provides a separate area of poorly oxygenated water for the de-nitrifying bacteria to thrive without depriving the fish and invertebrates in your saltwater aquarium of the oxygen they need to breathe.

Building a plenum yourself is one of the least expensive and easiest do-it-yourself projects in the reef aquarium hobby.

Before you begin, measure your aquarium bottom (length and width) so your plenum can be custom fit to your fish tank.

To make your own plenum you will need these hardware materials:

One roll of fiberglass window screen - the smallest roll you can buy should be enough.

Four-PVC elbow joints for joining one inch PVC pipe.

One sheet of construction eggcrate, the plastic material sometimes used to protect fluorescent light fixtures.

At least 20 eight inch cable ties (a.k.a. zip strips)

Two pieces of one inch PVC pipe three inches shorter in length than the length of your aquarium.

Two pieces of one inch PVC pipe three inches shorter in length than the width of your aquarium.

Three six inch long pieces of PVC pipe for each one foot of aquarium length - round up.

You should be able to find all of these materials at any Home Depot, Menards, or Lowes or any other large home improvement or hardware store.

You will also need these tools:

A pipe cutter - unless you can persuade your hardware store to cut the PVC pipe into the lengths you need.

Heavy scissors.

Tin snips.

To assemble the plenum:

Lay out the materials for your plenum in a flat workspace. Attach two of the PVC elbow joints to the ends of each of the two pieces of PVC pipe that are three inches shorter than your aquarium's width. Then attach the two longer lengths of PVC pipe using the elbow joints to create a rectangle of PVC pipe. This will serve as the frame for the plenum.

Hold the eggcrate over the PVC rectangle and cut it to the same size as the plenum frame using the tin snips.

Attach the eggcrate to the top edge of the plenum frame using 4 cable ties around the PVC and through the squares of the eggcrate material, one cable tie at each corner. Cut off any excess length of the cable ties using the tin snips after pulling them as tight as possible.

Attach the six inch long pieces of PVC pipe evenly spaced on the underside of the eggcrate using a single cable tie around the middle of each piece threaded through the eggcrate squares. Tighten the cable ties and cut off the excess length using the tin snips. When this step is completed you will have a piece of eggcrate attached on top of a rectangular frame of PVC supported by additional pieces of PVC pipe underneath.

Cut a piece of window screen large enough to completely wrap around the plenum frame and eggcrate with about an inch of excess on all sides using the heavy scissors. Wrap the window screening all the way around the unfinished plenum box and bring the edges of the screening together at the narrow ends of the plenum and along one long side of the plenum at the PVC pipe. Use the remaining cable ties to loop through the window screening, the edge of the eggcrate, and the PVC frame. You may need to cut small holes in the window screening using the scissors to fit the cable ties through. If this is necessary, don't cut the holes too close to the edge of the screen. Tighten the cable ties and cut off any excess.

Now your aquarium plenum is completed and ready to use. Simply place it in the bottom of your fish tank and layer the live sand or crushed coral on top of it before filling the tank with saltwater.

Sources:
Over twenty years in the aquarium hobby plus experience in marine aquaculture and saltwater fish breeding.