What Is The Point Of A Plenum?

Been awhile since I looked into these, but I remember thinking that it didn't make a whole lot of sense to me - it is essentially some big open space, but if its a big open space doesn't that mean there is little room for bacteria to grow with?

I did a 55 gallon with a deep sand bed, but under the sand is a ton of bio-media. In several places I placed some bio media under some tupperware with a few holes drilled in it to act as mini-plenums or what not. That tank has its water parameters stay WAY more stable than my other two that don't have sand/bio media beds, one has a HOB and the other a sponge filter. I always attributed to the deep sand bed/huge amounts of bio media, not so much the mini-plenums but I am not sure haha.

Thanks!

 

for anaerobic bacteria to consume nitrates. mainly used in saltwater tanks. can be used for freshwater too. the bacteria is in the substrate and not in the open space below.

The Plenum - Could It Work For You | Aquarists Online
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.

MrBryan723 has a setup with an under the substrate canister which pretty much accomplishes the same goal as a plenum
Nitrate Filtration Better Than Expected

 

Actually I did a test today. About 2 weeks ago I started putting my plant trimmings in the other tank to decompose hoping I would register some nitrates. But nope.

 

Been awhile since I looked into these, but I remember thinking that it didn't make a whole lot of sense to me - it is essentially some big open space, but if its a big open space doesn't that mean there is little room for bacteria to grow with?

The plenum is there to make it easier for the water to very slowly move through the substrate. It's not directly meant to harbor bacteria. That's what the substrate is for.

The substrate right on the tank bottom is a bit like a dead end street where the road just stops. There's no easy way to turn around and if you get many cars driving that way it can easily end up in a traffic jam as cars are trying to make three-point turns to get headed back the other direction. With a plenum it's more like a dead end street with a large cul-de-sac at the end. Lots of room for cars to easily turn around and head back out the other direction.

The water doesn't flow through the substrate as easily when it's sitting right on the tank bottom. With a plenum the water can move through, circulate around under the substrate, and then head back up. The difference in flow is actually very small, but still apparently significant. The bacteria we're trying to encourage need low oxygen water, not water totally depleted of oxygen. The plenum is supposed to help promote those conditions.