The word plenum is not a term which the majority of beginners have actually heard of especially when it comes to filtering the aquarium. The plenum however is very similar to that of a deep sand bed however both the creation and design are entirely different.
So what actually is a plenum?
A plenum is not a filtration unit but is actually a space which is located under the substrate and separates the substrate from the bottom of the aquarium.
This space (the plenum) is full of both the water and any organic waste as the water is diffused into it. A plenum is also sometimes referred to as a Jaubert system as Dr Jean Jaubert was the individual who discovered it.
The structure of a plenum is actually very very simple. It is basically a deep layer of substrate which is suspended above the bottom of the aquarium to create a space or a void if you will.
How it works is, although slightly scientific, also relatively simple.
As the water moves slowly through the substrate the nitrifying bacteria in the upper level use up all of the oxygen in the water and convert the ammonia to nitrite and then the nitrite to nitrate. The nitrate which remains is the transported with the water down towards the plenum. As the water reaches the lower levels of the substrate the nitrate is then slowly broken down into nitrogen gas by the denitrifying bacteria which live in the very low oxygen levels of the substrate.
The reason the water moves slowly through the sand is due to the anaerobic action being performed in the lower levels. The anaerobic actions produces heat which heats up the water in the plenum area. This heated water moves up through the substrate which therefore displaces the cooler water above it causing the water to move through the substrate at a very slow rate.
A slow rate of water movement is important in a plenum as it assists in the prevention of a dangerous build up of hydrogen sulphide.
The upper levels in the substrate will become home to various organisms which can either be introduced via the aquarist or are organisms which have relocated from their home in the live rock. These are very important, the same as they are in a deep sand bed. Their job is to keep the upper levels of substrate ‘turned over’ and clean from both detritus and waste.
The above is how a plenum operates but how do you actually make one.
Again this is relatively simple. Not as simple as a deep sand bed as it does require a bit of DIY but simple all the same.
The items listed below are required to make the plenum. This list is not set in stone and can be modified to suit your/the aquariums needs.
Marine safe PVC piping.
Egg crate to make the grid from.
Aragonite based sand with a granular size between 1 and 2 mm.
The recommended minimum required depth of substrate in a plenum is 2 ½ inches when placed on top of the plenum grid therefore you need to ensure that you obtain enough aragonite/live sand to accomplish this.
The first thing you will need to do is actually create the support which the substrate which sit on top of and create the plenum. Personally I find that marine safe PVC pipe is the best for this. These, when laid on their side provide a lot of strength for the substrate held above it.
I would recommend that you cut this piping in lengths of two inches or so and ensure that you have enough of them to support the weight of the substrate above it. Remember that you may also be placing rock on top of the substrate in the aquarium so make enough of them to support this as well. If you are unsure make more than you think you will need.
Once all the support have been kit you will now need to make the grid. The grid is basically there to hold the substrate up and also to prevent any of the substrate from falling down into the plenum area and effectively making it useless.
The easiest way I have found to do this is use eggcrate and cut this to the same dimensions as the inner dimensions of your aquarium. It does need to be noted that the grid needs to be a very tight fit so take your time cutting it. Once cut it can then be attached to the supports. I have found that tie wraps are excellent for this.
The basic structure of the plenum is not in place, however if the substrate was to be placed into the aquarium at this point it would simply fall through the eggcrate into the void. This is where the screen is used.
Cut the screen so that it is slightly larger than the inner dimensions of your aquarium. Once cut carefully place it into the aquarium, fold up the edges and carefully silicone it to the side of the aquarium. You will need to allow the silicone to cure which normally takes about 24 hours but once cured you should have a screen which no substrate will fall through.
Once cured you will be in a position to add the lower level of substrate. Add 1 ½ inches or so of aragonite and give it a really good stir using your finger to remove as many pockets of air as possible. Also be exceptionally careful not to tear the screen otherwise all your hard work will go to waste.
Now that the first layer is in place you will need to cut a second screen to slightly larger than the inner dimension of the tank, and silicone it in place as you did the first screen.
Once the curing process is complete put another 1 ½ inches of the aragonite sand into the aquarium. As with the first layer give it a really good stir with your fingers to remove as many air pockets as possible again being careful not to damage the screen.
Now we are ready for the live sand. Add about 1 inch or so of live sand to the top of the final layer – a screen is not required this time as it does not matter if the live sand mixes with the top layer.
The plenum is now complete and ready for water. When filling with water fill it slowly to avoid creating any air bubbles and mixing all the sand up.
So now we should have a plenum area at the bottom topped with a screen, on top of this screen is some aragonite sand followed by another screen. On top of this screen is some more aragonite sand and then the live sand. All of this is supported via some pvc piping to take the weight and create the plenum space.
In a plenum system it is recommended that little or no water flow be directed across the surface of the substrate. It is also recommended that a protein skimmer be utilised with a plenum as this will assist in the removal of any dissolved organic compounds which are not removed by the plenum.
It is easier to introduce a plenum to a new aquarium rather than the modification of an existing one. If you are interested in introducing a plenum to an existing system you will either have to remove everything from the display aquarium to introduce it or build a plenum in a separate aquarium and attach it to the system using overflows and pumps.
As with the introduction of any filtration device you will need to monitor the water parameters carefully so that you know that the nitrogen cycle is functioning properly.