# Ever heard of a plenum?



## mistergreen (Mar 3, 2007)

I just recently heard of it. It's a gap of space under the substrate. It's basically an under gravel filter but no uplift from an airline or pump.

The idea is to provide low O2. No O2 is when anoxic condition occurs. It would work for dirt substrate too if you layer a garden fabric so the dirt doesn't fall through.


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## TAB (Feb 7, 2009)

Those have come in and out of favor at least 5 times in the last 20 years for reef tanks. I would not want one in a planted tank as roots would go thru he plenum pretty quickly


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## mistergreen (Mar 3, 2007)

Why did it fall out of favor in the reef world?
A layer of the garden fabric would stop roots.


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## hoppycalif (Apr 7, 2005)

How does the water in the plenum get more oxygen when what is there is used up? I haven't tried it, but I don't see what problem it solves.


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## TAB (Feb 7, 2009)

Everything does. 20 years ago turf algae scrubbers was the hot ticket. Them came more powerful skimmers, then bigger skimmers and nitrate reactors, then chemicals, the fuges.... guess what they are back on? Turf scrubbers only they call them " algae reactors" or " algae chambers" the only thing new is they are now vertical instead of horizontal and they use led lights not vho


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## mistergreen (Mar 3, 2007)

From what I understand,O2 levels decrease as you go down the substrate but it doesn’t go down to zero with the plenum. In low O2 environment, different nitrifying (denitrifying) bacteria consume NO3. Since O2 is low, they take O3 from NO3. By product is N2 gas. They do need carbon for energy. Sounds a lot like a compost pile.


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## ChuckM (Mar 19, 2020)

mistergreen said:


> I just recently heard of it. It's a gap of space under the substrate. It's basically an under gravel filter but no uplift from an airline or pump.
> 
> The idea is to provide low O2. No O2 is when anoxic condition occurs. It would work for dirt substrate too if you layer a garden fabric so the dirt doesn't fall through.


I have two tanks, a 65 and a 6 (Fluval Edge) with plenums under a deep substrate of a mix of Oil-Dri (baked clay absorbent) and Eco Complete (about 3:1). The 65 gallon has a 3" bed with a 1" plenum and the Edge has a 1.5" with a .75" plenum. These tanks are 2 years old, moderately stocked with fish, snails, shrimp and plants. Both have HOBs.

My thoughts on the process came from reading up on Jaubert, Novak and the video series from Jay's Aquarium which I highly recommend for a plain explanation of the anoxic denitrification process, which is the whole point. From a reef perspective (which I'm not), I also drew on the GARF group who advocated a more open plenum system meaning more water circulating. The freshwater systems I'm experimenting with only have access via migration through the substrate and separating mesh.

In retrospect I'm not sure a plenum is needed, maybe only the deep substrate but I'm a believer in anoxic conditions for denitrification. My nitrate levels run around 20-30 ppm which I find very acceptable. I don't do a lot of water changes, maybe 40% once a month or so, and I don't do a lot of gravel vacuuming as a lot of waste breaks down naturally and is absorbed into the substrate. I'm on a well and use that water from the faucet. It's by then passed through a Whirlpool household water softener and a GE whole house filter.
My plants and animals are thriving and healthy.


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## mistergreen (Mar 3, 2007)

yeah, It sounds like a plenum isn't needed in a planted tank. The plants will handle any nutrients. I think it might be good for anoxic dead spaces in the substrate, especially under large rocks and driftwood. 

I have a small goldfish pond with a gravel bog filter with plants. It gets water changes when it rains. Last time I tested the NO3, it was 0ppm.


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## mistergreen (Mar 3, 2007)

I'm curious if a plenum could resolve anaerobic/anoxic situations.


















About 2 inches of top soil and 2 inches of sand, about 1 inches of space beneath the substrate.


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## mistergreen (Mar 3, 2007)

*9 day update*
There are bubbles in the substrate but doesn't smell of sulphur, probably CO2 or trapped air.
It has been 90F around here. O2 can't be too much in this container.


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## mistergreen (Mar 3, 2007)

Forgot to make a control for this experiment where there is no plenum and I expect it to go anaerobic. Will use the same material and substrate depth.


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## adalah (Feb 10, 2019)

chuckm said:


> i have two tanks, a 65 and a 6 (fluval edge) with plenums under a deep substrate of a mix of oil-dri (baked clay absorbent) and eco complete (about 3:1). The 65 gallon has a 3" bed with a 1" plenum and the edge has a 1.5" with a .75" plenum. These tanks are 2 years old, moderately stocked with fish, snails, shrimp and plants. Both have hobs.
> 
> My thoughts on the process came from reading up on jaubert, novak and the video series from jay's aquarium which i highly recommend for a plain explanation of the anoxic denitrification process, which is the whole point. From a reef perspective (which i'm not), i also drew on the garf group who advocated a more open plenum system meaning more water circulating. The freshwater systems i'm experimenting with only have access via migration through the substrate and separating mesh.
> 
> ...


+1


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## mistergreen (Mar 3, 2007)

*15 day update*
I saw little bubbles in the soil, looks like fermented beer or dough. There is a small hint of sulfur, nothing major. I don't know if this is possible but there is a small hint of alcohol as well.


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## Squawkbert (Jan 3, 2007)

My take on the whole plenum concept -
It takes up space, the potential downside is way larger than the benefit it might provide (until a sulfur pocket nukes your livestock).

The main benefit is supposed to be that you get lower nitrates.
There's an easier, safer way to do this...
Just run a really, really long (like 10m or more) piece of airline tubing from your aquarium into a nice dark, dark, completely sealed from light, darkroom quality kind of dark box, where coils and coils of tubing sit in total darkness.
Then you hook the other end to a low flow rate peristaltic pump that draws from the tank. If you do it right - right flow, right tubing length, darkness, you will eventually have anaerobic NO3 fixing bacteria in the box of tubing. There's no mulm to convert to H2S, it just lowers your nitrate. If it goes too low (<20 or 30ppm), you either shorten your amount of tubing in the box a little or increase the flow rate of the pump a little.


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## hoppycalif (Apr 7, 2005)

There is no reason to concentrate at all on keeping nitrates low. Many people have demonstrated that 80ppm and even higher does not harm their fish. In fact, people who use high light and CO2 also dose nitrates, because they are the first nutrient to run low in that situation. Plants use nitrates, and a well planted tank cannot experience too much nitrates. We used to worry all the time about too much phosphates, but now, for some reason, we worry about nitrates. Growing plants use up both of those nutrients.

I'm just blowing off steam!


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## Squawkbert (Jan 3, 2007)

hoppycalif said:


> There is no reason to concentrate at all on keeping nitrates low. Many people have demonstrated that 80ppm and even higher does not harm their fish...


A few years back, my reading indicated that having some nitrates was no problem but having either too much (hundreds of ppm) or too little (<40ish) could be detrimental.
Frankly, I'd rather have live plants than the contraption I mentioned above. If, however, you're a keeper of fish that won't allow plants to prosper, a "denitrator" may be worth considering if nitrate buildup is what dictates your water change schedule. At least, that was the thinking the last time I was anything near "current" on such things.


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## mistergreen (Mar 3, 2007)

*20 day update*
Fermentation smell is down, very little sulfur smell. 

No smells in the control, 10 days in.


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## mistergreen (Mar 3, 2007)

hmm. My control isn't going anoxic. I'm going to have to add sulfates into the experiments. I have some MgSo4.


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## ChuckM (Mar 19, 2020)

mistergreen said:


> hmm. My control isn't going anoxic. I'm going to have to add sulfates into the experiments. I have some MgSo4.


How are you seeing that it's not become anoxic?


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## mistergreen (Mar 3, 2007)

ChuckM said:


> How are you seeing that it's not become anoxic?


Using the best instrument to detect H2S, the nose.


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