Building and Using a Plenum

[CollegeReefer]

There are many ways to set up a saltwater tank sand bed and each way has it benefits. The 3 common methods are a shallow sand bed, a Deep sand bed, and a plenum. The shallow sand bed is great for the beginners where as the deep sand bed and plenum are more of an advanced set up. I would like to look at using and setting up a plenum.

I was first introduced to plenums by garf.org. All of there systems are set up using plenums. They swear by them and so I thought I would give it a shot. A plenum creates a space under the sand which causes a place for water to sit with little oxygen, an anoxic zone. You will see a picture bellow. I looked at other web forums and journals and there seems to be a never ending debate whether a plenum or deep sand bed is better. The philosophy behind both the plenum and a deep sand bed is to provide a place for denitrofrication to occur, specifically ridding the aquarium of nitrates. The bacteria needed to do this needs little to no oxygen and needs it completely dark. The heat the bacteria makes while ridding the water of nitrates cause the water to rise. The colder water from above sinks down in its place. Nitrates are the hardest to get rid of and using a plenum or deep sand bed will take care of most of the nitrates. This is just the basics of the plenum, but if you would like to know more just ask or look it up online.

I set up my plenum using the plans from garf.org. The can be found here. Setting up a plenum is very easy, for all it requires is egg create, pvc pipe, window screening (not metal), and zip ties. Here is a picture of the how it looks like on paper and what it looks like after assembly. As you can see it is very simple.



You can see the first layer of black screen around the whole unit


When you place it into your tank you will want to make sure there is a little space on all sides. This is so that you can not see the pvc and to prevent an light from entering the void. You will also notice the riser tube. Garf runs the plenum like a undergraval filter till the tank has cycled and the takes the tube out or cap it off. Some other designs do not use the riser tube.





Garf plans suggested that you put about two inches of substrate on top of the plenum and then add another layer of screening to keep burrowing critters from disturbing the lower sand. Once again this is not done by all, but because I was going off of garfs plans I did it.





You put another three inches of substrate on top of that second screen. In the end there is about 5 inches of sand from the bottom of the tank including the plenum.





I currently have the tank cycling and once it is done cycling I will take out the riser tube and put in a smaller capped tube to keep out the substrate. This tank will be a new frag tank and I will keep you all informed how it does. If you have any questions, comments, or just want to add something I missed, please post.

[musho3210]

bb? (bare bottom, nothing to do with plenums but bare bottoms are common)

[CollegeReefer]

Bare bottom is used more with qt tanks and frag tanks, but i guess you could do it.

[musho3210]

ive seen a lot of bb sps tanks since most sps tanks has a very strong flow and with strong flow they can take advantage of the bb effect.

[Bear]

so right now there is a powerhead connect to the rise tube, how come it doesnt pull up any sand from the plenum.

so ur going to take out the tube connected to the powerhead and replace it with a capped tube? why? Whats the pointed in having the powerhead there then?

I guess ill read up a lil

[musho3210]

i think (i dont know) the powerhead is meant to provide nitrate rich water in the early stages to the plenum

[CollegeReefer]

Originally Posted by musho3210

i think (i dont know) the powerhead is meant to provide nitrate rich water in the early stages to the plenum

Right on. The tank needs to cycle and the bacteria levels need to get up to par before caping the pipe. I think it is garfs and others belief that this speed up the process a bit rather then just letting the water sink to the bottom.

The pipe is going down into the unit itself where there is no sand. The thing i built, which is the actually the plenum itself keeps all sand out of the area. The pipe is going to a bare bottom. It is this void where all of the nitrifing bacteria can thrive and remove the nitrates.

[musho3210]

i have a question, could you put some ceramic rings or other high surface area stuff down in the gap/plenum/whatever it is I would think it would give the de-nitrifying bacteria more area to live on and in turn have more.