# [Wet Thumb Forum]-plenum for denitrification



## Kurt Reinhart (Mar 4, 2003)

In line with discussion on creating natural/low maintenance tanks, I've been looking at some methods for designing low maintenance reef tanks. It seems the use of deep sands with a plenum (essentially a water gap created below the sand which becomes anaerobic) is used to have bacteria convert NO3 into O2 (for oxygen) which releases N2 as a gas and completing the nitrogen cycle. Reef keepers seem to think this works if tanks are kept with few fish (low Nitrogen inputs). It sounds like they are able to keep all forms of nitrogen at ppm's close to zero. Do you think that a portion of a freshwater planted tank could include a plenum and that this would reduce Nitrate buildup? I know the aneorobic soils are not good for plant roots so the tank would need to be divided into a planted portion and an unplanted portion with a plenum.

I'm sure that small levels of denitrification occur in planted aquariums, but this might make our tanks even more self sufficient. It seems we might be able to learn from reef keepers and their attempts to more accurately mimic the functions in natural systems. "The Natural Reef Aquarium" is a good book that describes the dramatic changes in reef aquarium designs from tanks with no soil/sand (requiring lots of high tech equipment, water changes, etc.) to tanks with deep soil/sand (less equipment and fewer water changes).

Food for thought.


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## Kurt Reinhart (Mar 4, 2003)

In line with discussion on creating natural/low maintenance tanks, I've been looking at some methods for designing low maintenance reef tanks. It seems the use of deep sands with a plenum (essentially a water gap created below the sand which becomes anaerobic) is used to have bacteria convert NO3 into O2 (for oxygen) which releases N2 as a gas and completing the nitrogen cycle. Reef keepers seem to think this works if tanks are kept with few fish (low Nitrogen inputs). It sounds like they are able to keep all forms of nitrogen at ppm's close to zero. Do you think that a portion of a freshwater planted tank could include a plenum and that this would reduce Nitrate buildup? I know the aneorobic soils are not good for plant roots so the tank would need to be divided into a planted portion and an unplanted portion with a plenum.

I'm sure that small levels of denitrification occur in planted aquariums, but this might make our tanks even more self sufficient. It seems we might be able to learn from reef keepers and their attempts to more accurately mimic the functions in natural systems. "The Natural Reef Aquarium" is a good book that describes the dramatic changes in reef aquarium designs from tanks with no soil/sand (requiring lots of high tech equipment, water changes, etc.) to tanks with deep soil/sand (less equipment and fewer water changes).

Food for thought.


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## Neal (Mar 1, 2003)

Its an interesting idea, the part I don't like is giving up space on the bottom of the tank for the denitrification. Perhaps one of the coil design nitrate removers with slow flow could substitute for the plenum?

Also I don't think nitrate buildup is that big an issue in a low tech Walstad style tank, unless you have heavy fish loads. I could be wrong though I've never had one set up for longer than a few months.


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> It sounds like they are able to keep all forms of nitrogen at ppm's close to zero.


That might not be a good outcome in a planted tank.



> quote:
> 
> Do you think that a portion of a freshwater planted tank could include a plenum and that this would reduce Nitrate buildup?


Lots of planted tanks have or have had plenums. Usually we just think of them as unused undergravel filters. Planted tanks don't generally have an issue with nitrate buildup so it's hard to say what role the "plenums" might fill.

Nitrate buildup can happen in slow-growth tanks with inorganic substrates. Apparently it does not happen in Diana Walstad's tanks. I talked with her at the AGA conference in Dallas. She believes that she was able to show experimentally that the lack of nitrate buildup in her tanks was due to denitrification in the soil substrate.



> quote:
> 
> I know the aneorobic soils are not good for plant roots so the tank would need to be divided into a planted portion and an unplanted portion with a plenum.


Check your sources on the idea that anaerobic substrates are bad for plant roots. Most natural aquatic substrates would be anaerobic and plants grow very well in them. Plants also seem to grow well in anaerobic aquarium substrates.

The plenum -- if present -- will not create anaerobic conditions. If anything the plenum will moderate conditions so that the oxidation potentials don't get real low.



> quote:
> 
> I'm sure that small levels of denitrification occur in planted aquariums, but this might make our tanks even more self sufficient.


Plants consume nitrate through assimilative denitrification and they can use quite a bit. Bacteria can use even larger amounts of nitrate through nonassimilative denitrification. All told, large amounts of denitrification can occur in planted aquariums.



> quote:
> 
> It seems we might be able to learn from reef keepers and their attempts to more accurately mimic the functions in natural systems.


That used to be a fairly common idea. I think it's wrong. The ecology and biology of a marine reef is vastly different from the ecology and biology of almost all freshwater environments. Rocky offshore areas of great lakes are the only possible exception that comes to mind. If you were to try to pattern a freshwater tank after the systematics of a marine reef the result would be distinctly unnatural.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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## tdog44632 (Feb 5, 2003)

In reefkeeping an important part of the denitrfication is animals that keep the sand stirred so nitrogen can buble up while nitrate difuses down. in saltwater lots of sand stars copepods amphipods conchs and sea cucumbers available to the hobbyist to facilitate this process. freshwater does not have this avalable, although you could possibly collect for yourself. the plenum has been found to not be neccasary. Dr. Ron Shimek is the expert in sandbeds, you can ask him yourself at reefcentral.com( first you have to register) 

FWIW,

Tony


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## Roger Miller (Jun 19, 2004)

There's nothing unique about the stirring role of substrate dwellers in reef tanks. Malaysian trumpet snails fill that role in freshwater tanks very well.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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## tdog44632 (Feb 5, 2003)

True that malaysian trumpets snails stir substrate, but usaually they stir large grain size substrate. Should have mentioned that grain size is important for DSB, the best DSB are almost mudlike because of differing grain sizes. Small size is important because of the increased surface area for denitrification. Hobbyists often use playsand( silicate based) or preferably Southdown sand( calcium based). The grain size would be too small for MTS to stir(correct me if I am wrong). the DSB would stratify and become usesless. Sea Cucubers actully ingest the sand to strip excess material off grains to maintain surface area for denitrifiation.

Tony


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## Roger Miller (Jun 19, 2004)

I have active populations of MTS stirring substrates with fine sand. Fine grain size doesn't seem to keep MTS from stirring it about. I've even had healthy populations at work in heavily organic substrates. Freshwater clams can also move quite a bit of material. If you want more active stirring then there are quite a few fish that will happily dig into and sift through a substrate. The larger ones are pretty disruptive to plants.

If you actually wanted a freshwater tank to get as close as possible to the little stirrers that turn over marine sand then you would need to get a population of chironomids, other insect larvae and worms. Most of us don't really want to do that.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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## codeflag99 (Mar 19, 2004)

> quote:
> 
> If you actually wanted a freshwater tank to get as close as possible to the little stirrers that turn over marine sand then you would need to get a population of chironomids, other insect larvae and worms. Most of us don't really want to do that.


Insects, as in _flying_ insects!

I personally can't really warm up to the idea of a lot of creepy-crawlies in my substrates, and I know my wife won't like the idea of various bugs flying out of the aquarium into the house.









As far as DSB, many planted tanks have substrate depth that would qualify as DSB in the marine side of the hobby (3-4" +).

Based on what I've been reading, the DSB method (without a plenum) works just as well.

I have never tried a plenum in a planted tank, however, I have a Central American cichlid aquarium with one. Is does help keep the nitrates down.

If anyone is interested here is a link that talks about freshwater plenums. Remember when reading that they have something to sell. I have never used their filters, but I have had a look at them, and seen them running in stores. By all accounts they do work. I guess it saves the effort of figuring out depth, sand grain size etc., on your own, but I prefer DIY myself.

::: FishNeeds.Com ::: HappyFish™ Plenum System ::: Information Articles >>>


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> Originally posted by codeflag99:
> Insects, as in _flying_ insects!


Exactly. And that gets to the big difference between reef systems and natural freshwater systems that makes it so difficult to transfer reef methods to a "natural" freshwater tank.

Reefs are substantially isolated systems that have little interchange with other environments; nutrient levels are low, nutrient flux is low, water clarity is high and the food web is largely self-contained.

Freshwater bodies are intimately related to the terrestrial environment around them; wetlands and most small water bodies of freshwater aren't even distinct from the surrounding terrestrial environment. There is a huge amount of interchange. Nutrient levels are high, nutrient flux is high, water clarity is variable and often low and only a small part of the food web is contained in the aqueous environment.

To keep a natural freshwater tank it would first and foremost not be an aquarium. It would be a paludarium. The largest part of the faunal biomass would be insects and insect larvae. A bird or two would be a good addition, as would be some voles. The aquarist could follow common practice of feeding a freshwater tanks with dry foods and siphoning or filtering the debris. That process is a good analogy for the flux of nutrients through a freshwater environment.

It could all be very interesting to do, but there would be precious little learned by reef keepers that would do you much good.

Roger Miller

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_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


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## Kurt Reinhart (Mar 4, 2003)

Great comments. I learned alot. My tank is ~100 gallons and has native fish, two turtles (diamondback terrapins fed outside of tank), and two java ferns. With the additional animals, no soil, few plants; the nitrate is more of a problem than the planted tanks that most of you keep. But still not much of a problem. My tank isn't exactly a planted tank and that is largely because I will be moving in 1.5 years. Then I plan to build an even larger tank (same general design as 100 gal) that I plan to heavily plant as a Florida biotope. ANyway, it was helpful to hear your comments.

I should clarify an original comment. I didn't mean to imply that we should try and make freshwater tanks like reef tanks but that we should consider approaches to tank design that consider natural processes.

Also, I think that many of the spring systems in Florida would be considered very stable in terms of temperature, clear water, and probably even nutrients although nutrient inputs would still be greater and vary more than reefs.


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## tdog44632 (Feb 5, 2003)

Using insects is exactly what I meant. You would probably collect by yourself and DSB would probably work. Having fish that dig would probably be bad though if they dug through to the hypoxic and anoxic layers and disturbed the de-nitrification process.


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## Rapier (Mar 30, 2004)

The plennum method is and should only be used in reef keeping. 1 becouse of the high sand bed required about 6" to 7". The difrent grain sised sand needed.and the microfauna needed for the sand bed.

i want to ask to though could you keep plants with a 50\50 10000k and actinic 03???


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## Wally (Aug 31, 2003)

Hello all,

logging into this topic a little late, (tax season at work very busy) anyway I have been experimenting with this idea for about 3 years now I guess and I have learned quite a bit.

Deep sand beds actually work quite well in planted tanks, and I think it makes the plants grow better. In my tanks set up this way I use about 4-6 inches of Quickcrete play sand which sits on top of about 1 inch of plain kitty litter.

I find that Malaysian Trumpet Snails work quite well in keeping the sand stirred in open areas, but they are not needed in tanks that are loaded up with plants that are "heavy root plants" like sags, crypts, vals, swords and most stem plants.

I also think that the plant roots actually send down quite a bit of oxygen down into substrate. You can see this quite well in my tanks that have been set up for a year or more. What has happened is that you can clearly see rust around the plant roots down in the very iron rich kitty litter. I would also like to add that I have no black areas visable that you would think I would have everything is as white as the day I set it up. I recently tore down a 29 gallon that had been up and running for almost 2 1/2 years and it was clean sand all the way down to the rusty kitty litter.

I think the reason that this works so well is totaly different from why it works in a reef tank, I think it mimics very closely the natural substrate that plants grow in around the world.

I should also add that the tanks that I use this in are far from being amano type tanks. Most of these tanks are set up with the kitty litter and sand planted with vals or sags and let go. I think if you are going to uproot and move plants around quite a bit that this is not the way to go, if however you are looking for a jungle of plants, and zero algae that this way rocks.

*Don't feel like Satan, but I am to them...*


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## Kurt Reinhart (Mar 4, 2003)

Hey Wally,
Sounds interesting... Do you have any pics of your tanks up on the net? If so where? If not would you consider posting them. I would like to see how well you plants are doing since I'm considering planting a tank with a lot of Sag.


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## imported_RTR (Sep 11, 2003)

I've been playing with FW plenums since late '98, and for me they do function as denitrification beds without the reef assortment of diggers, but MTS are present in most of of my tanks and in all the plenums. I doubt they dig full depth, I'd credit roots with greater effects. For the heavily planted plenums it is really hard to tell how much denitrification is going on. My tanks are only moderately lighted and generally not CO2 supplemented (one plenum is supplemented), so tanks such as an all-Anubias tank would build up some nitrate conventionally handled, while over a plenum it does not - but that is not quantification, only a hoped-for result which was not the case for a similar tank without the plenum.

Of the five I've set, I took three down and reset conventionally as part of some tank moves and resets. One of those was the unplanted control, which never had detectable nitrate after the 8th month by common hobby kits (La Motte would show low ppm periodically). If I still kept a number of big nasties, I might set such a tank for them, but I don't, so it serves littl real purpose for me. The planted plenums other than the Anubias tank needed nitrate supplement, but they were heavily planted so this is no shock to anyone. The two remaining I am likely to hold for the originally projected ten-year span, but I don't think I see much point in special denitrification in planted tanks - none went to the point of iron precipitation or sulfide generation, but neither do my routine deep but low-organic substrates.

I agree with those who think that some denitrification occurs in many deep substrate tanks independent of plant uptake, and is pretty much a "so what?" situation. I also agree that roots are having a major effect on substrate redox, such that comparisons to SW beds with or without plenum are far-fetched. Even if the net result is similar, the process is very different.

"Where's the fish?" - Neptune


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