Reef Plenum: Part 1

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August 2002:
I have used under gravel filters (UGF) since I first started with freshwater aquariums. There seems to be a lot of debate over UGFs on the internet. From my experience, the UGF is a great asset to your tank. I have never had a problem with water quality or the health of my fish. So naturally, when I decided to built a reef tank, I decided to add a plenum.

What is a plenum you ask? A "plenum" is an air space. In an office, it is the area above your ceiling tiles (hence "plenum rated cabling", or cabling that can be installed in an airspace that will not be poisonous if it burns). In an aquarium, the plenum is a space under the substrate, which separates your sand bed from the bottom of the tank. This space becomes filled with water and organic waste in the water gets transported into the plenum by way of diffusion. As the water goes through the sand, nitrifying bacteria in the upper layers of sand utilize the oxygen in the water and break down the organic waste from Ammonia, to Nitrite, and then into Nitrate. The Nitrate is then transported into the plenum via the oxygen reduced water. The Nitrate concentrates in the plenum and slowly gets broken down into harmless chemicals by denitrifying bacteria that live in the oxygen poor environment of the lower levels of the sand bed.

In conjunction with my protein skimmer, UV, and wet/dry filter, I should have a very efficient filtration system. If I don't, then I am wrong, but I still had a great time building the system and will learn from my mistakes the next time around.

Building a Plenum for under $10:

Most websites advocating plenums suggest that you use "eggcrate". Eggcrate is an interwoven waffle-like plastic sheet. Take a look up at your office's fluorescent fixture. Now think of the criss-cross metal protecting the bulbs being much smaller. You can purchase eggcrate from Home Depot in the lighting section. But it usually only comes in white and costs about $10-$20 for a 24"x12 (to 48") sheet. I took a drive down to my local day-old bread store and checked out the trashcans out back. I came away with about seven 24"x48" bread crates that were perfect. It is important to note that I took these from inside the trash bin and not their loading docks. These were trash and not awaiting pickup.
I then measured out my tank, including the opening at the top. It does you no good to have these big crates that you cannot fit into your tank, or that will be impossible to remove if you get them in. I settled on slicing them in half, the long way - ending up with a 12"x48" rectangle. Since my tank is 19"x59", I then needed a slightly shallower bit for the rear of the tank. All of this was cut with my handy-dandy Black and Decker Versa-Pak 7.2V, 3-3/8" circular saw. In about 30 minutes (give or take a couple measurement mistakes) - - pick up extra crate if its there! Notice the notch that I cut for the pre-filter. You also do not want to cut the crates to exact bottom of your tank. Leave some space so they do not "wedge" into position. Additionally, you need to leave a little space for the screen to wrap over, plus you want to have some sand/crushed coral between the front of your tank and the actual plenum. You don't want to look into the tank and see the crate.
Next I purchased two 10 foot 3/4" PVC (sch40) irrigation/sprinkler pipe. The thinner pipe runs about .96 cents at HD, with the SCH40 running $1.60. This was then used in conjunction with zip-ties to attach to the bottom of the crates to provide a uniform depth and rigid support. The 90º and "T" connectors for the pipe will be used later when I build and internal support structure for my live rock. I used zip-ties to attach lengths of cut PVC pipe to the bottom of the crate. I let a couple pipes "overhang" the larger crate so that they will support the smaller (back) crate when installed. Lastly, I drilled out (top, bottom, sides) of the pipe supports to allow water to flow through them. Next, we assemble the whole thing! Go-->