# Undergravel filters



## foxfish (10 Dec 2009)

I was hoping we could discuss UGFs.
For many years it was made very clear though magazines & other literature that UGFs were of no of no use in a planted tank. 
It was said UGFs would draw oxygen over the plant roots & stunt their growth. In fact I believe certain manufacturers of related plant growing equipment still print this though their advertising.

However it would seem that plenty of people have successfully used UGFs.....

Because I was always lead to believe UGFs would simply not allow my plants to survive, I have never tried using a UGF in a full on planted tank but I can see how they might be a benefit...

I would consider using a reverse flow system, pre filtered through a canister & then through a external reactor.

I would like to know if drawing Co2 enriched water up though the substrate would effect the plant roots?

I can see that this way the gas would be very evenly distributed & the water column would continuously fertilize the substrate.
Fox.


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## dw1305 (10 Dec 2009)

Hi all,
The main "problem" with a UGF would be that it is unlikely to function properly as a filter in a planted tank. If you fertilise the water column the substrate becomes relatively unimportant for many plants as a nutrient source, it is there purely to anchor the plant. The levels of oxygen in the substrate can't stunt the plant root growth, the root respires like any other tissue and needs a source of O2, it doesn't photosynthesise so the concentration of CO2 is irrelevant until the sediment becomes O2 depleted. Plants have various adaptations to allow their roots to survive in CO2 rich, (usually from the decay of organic matter) hypoxic substrates. The interactions between the root, rhizosphere and substrate are quite complex. This is a quote from Li & Jones 1995 paper "CO2 and O2 transport in the aerenchyma of _Cyperus papyrus_ L."  â€œâ€¦..While the water surrounding the rhizomes remained strongly hypoxic, the O2 concentration in the submerged rhizomes was 15.1% during the day and 10.3% at night".

In an unfertilised tank with a properly functioning UGF highly oxygenated water in the substrate will provide O2 to allow the bacteria to decompose any organic matter present, oxidisation reduces the availability of most ions, and loss of OM reduces the CEC of the substrate and make making the small amount of nutrients present less available to the plant. This often leads to the plant partitioning its limited resources into producing a large root system (to give it the best chance of retrieving an nutrients present ), and relatively little growth of shoots and leaves. This is why UGF's and plants are thought not to get on, but it is irrelevant in planted tanks where the water column is fertilised.

Having a UGF will limit the extent of any anaerobic sediment, (and stop the NO3 being de-gassed to N2 for example). Whether this a good thing or not depends on your point of view, personally I'd prefer an undisturbed substrate and I think these natural processes are both a good reason for both:

Â·         having a substrate, and for
Â·         leaving it relatively undisturbed.

Using nitrogen as an example, the uppermost surfaces of the substrate (both with and without a UGF) offer a potential home to the community of aerobic bacterial that metabolize ammonia to nitrite and then nitrate, (because the nitrification process uses a lot of oxygen). However only a few centimetres below the undisturbed substratesâ€™ surface, the diffusion of oxygen can't supply enough oxygen, and as oxygen levels fall anaerobic bacteria become more frequent. Much of the bacterial community is this zone are â€œfacultative anaerobesâ€; when oxygen is in short supply, they are able to switch to a metabolism that doesn't require oxygen, instead, they use nitrate, stripping the oxygen and leaving nitrogen (N2) gas. The nitrifying bacteria provide the nitrate, and their high oxygen demands also tend to exhaust the limited supply of oxygen. These two types of bacteria will occur across a fluctuating boundary lying not far beneath the surface of the substrate. The same processes will also occur in the â€œrhizosphereâ€ the aerated zone lying around aquatic plants roots. 

There is a lot more about the microbial dynamics of sediments and how they interact with oxygen and nitrogenous compounds on the â€œSkeptical Aquaristâ€ http://www.skepticalaquarist.com/docs/nutrient/wincol.shtml, and also on this page from Edinburgh University http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/winograd.htm.

Li M, Jones MB. (1995) CO2 and O2 transport in the aerenchyma of _Cyperus papyrus_ L. Aquatic Bot. *52.* 93-106.

cheers Darrel


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## YzemaN (10 Dec 2009)

Hooray! I absolutely love this kind of answers. It proves that UKaps is a world class website for plant geeks. Very informational and educational.  

UKaps Baby!


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## foxfish (10 Dec 2009)

Thanks for the reply - I am, however, not sure if anyone who has forked out Â£100 on a complete substrate would agree with you opinions - "If you fertilise the water column the substrate becomes relatively unimportant for many plants as a nutrient source, it is there purely to anchor the plant."-  :? 
I do realise that if you flow o2 rich water through the substrate there will be little if any (depending on the structure of the substrate media) anaerobic activity but I am not sure if this matters?
I was thinking about a more open structure for the substrate like a porous baked clay (cat litter).
I do think that by feeding Co2 rich water up through the substrate  (reverse flow UGF) would give a very even distribution of the C02 & offer a high turnover rate with out too much surface gas exchange.
Obversely this method would supply the roots, whether they need it or not, with all the fertilizers we feed into the water column.

I have read about UGfs being used very successfully on some American forums although some seem to use only the front half of the tank & others only a center space!


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## Dave Spencer (10 Dec 2009)

Are anaerobic areas common in our tanks, Darryl? I was under the impression that this wasn`t the case, but it is something I am not too sure of.

As for the nutrients in the substrate becoming irrelevant with a fertilised water column, I feel there are many that get better results with a high CEC substrate and water column fertilisation, as opposed to an inert substrate. Having said that, I am currently of the mind that expensive substrates may not be worth the money in relation to the improvement they bring, and have will be moving on to graded gravels when my Amazonia is exhausted. 

Dave.


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## Fred Dulley (10 Dec 2009)

Dave Spencer said:
			
		

> Are anaerobic areas common in our tanks, Darryl? I was under the impression that this wasn`t the case, but it is something I am not too sure of.



I have been under the impression that creating anaerobic areas to convert NO3 to N2 gas is rare and tricky to achieve in freshwater aquariums, usually causing more bother than it's worth.


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## foxfish (10 Dec 2009)

Dave Spencer said:
			
		

> Are anaerobic areas common in our tanks, Darryl? I was under the impression that this wasn`t the case, but it is something I am not too sure of.



I have been under the impression that creating anaerobic areas to convert NO3 to N2 gas is rare and tricky to achieve in freshwater aquariums, usually causing more bother than it's worth.[/quote][/img]




It can be done


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## Brenmuk (11 Dec 2009)

Why bother converting NO3 to N2 when NO3 is so important to plant nutrition?

My first tank had an UGF and I could never get plants to grow. Though looking back now they never had much of a chance as the gravel and water column was probably lacking in a range of nutrients and the air pump probably did a great job of driving off any CO2 that got into the water so I'm not sure now that the UGF was totally to blame.

Maybe you could make an UGF work with pressurised CO2 and water column ferts? 

But surely as plants grow (assuming you could get them to grow) their roots would clog up the UGF thereby reducing flow and efficiency of the UGF?


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## dw1305 (11 Dec 2009)

Hi all,
Yes a bit of a quick reply so "less haste, more speed". 





> I am, however, not sure if anyone who has forked out Â£100 on a complete substrate would agree with you opinions - "If you fertilise the water column the substrate becomes relatively unimportant for many plants as a nutrient source, it is there purely to anchor the plant."


 I should have said that some plants, Cryptocorynes for example, will do better with a  nutrient rich substrate, and that a complex natural mix of clays and humus may offer some small advantages in terms of growth and nutrient retention & availability over a less complex mix of (calcined) clays and humus (laterite, Akadama  and sphagnum peat or "kitty litter" and leaf mould). In both cases assuming that the mix has a reasonable CEC they will perform the same function. I now use a 80% silica sand/10% clay/10% leaf mould based substrate. This is because I am interested in limiting nutrients, if I was going for maximal plant growth I would use a more nutrient and humus rich substrate (probably by adding "Osmocote" or similar controlled release fertiliser, and more clay and humus), as well as adding CO2 and nutrients to the water column (as EI or similar).


> Are anaerobic areas common in our tanks, Darryl? I was under the impression that this wasn`t the case, but it is something I am not too sure of.


 The answer to this depends upon 3 factors, the nature of the substrate (depth, particle size, physical structure of the particles ), how highly oxygenated the water in contact with the substrate is and the BOD (Biochemical Oxygen Demand) of the whole system. A further factor is the extent and penetration of the roots (and rhizosphere) of the plants. 
If you have a thin layer of large spherical inert substrate (think marbles or rounded flint gravel) it will have no CEC and minimal biofilm development and it will remain fully oxygenated unless the water flowing through it becomes de-oxygenated (realistically by the tank inhabitants/chemical oxygen demand/ dead fish etc. before it reaches the substrate). In most(all?) other cases their will be zones of oxygen depletion within the substrates, and these zones will fluctuate dependent upon the factors mentioned (for example the substrate may become much more widely anoxic at night, when the tank water is less highly oxygenated.) If you have a substance with a lot of surfaces that the microbial biofilm can colonise, over time the lower layers of the film will be composed of anaerobes and facultative anaerobes.


 
This is from the Wikipedia article on trickle filters for waste water treatment http://en.wikipedia.org/wiki/Trickling_filter, but it is exactly the same concept. We are now in realms of the "redox value" of the substrate, and it is these "reduction and oxidation reactions" that regulate nutrient availability within the substrate.


> I have been under the impression that creating anaerobic areas to convert NO3 to N2 gas is rare and tricky to achieve in freshwater aquariums, usually causing more bother than it's worth.


 This is using the coil "de-nitrifier" shown in "Foxfish's" image. In these oxygen rich water is forced to spiral down through the layers of plastic coil tubing into the center of the cylinder.  As the water level increases within the body of the unit, the biological media ("bioballs" or similar) become host to bacteria. Th idea is that the O2 is consumed by the aerobic filter bacteria, but somewhere down the coil the O2 is exhausted and facultative anaerobic bacteria develop denitrifying the NO3 into NO and eventually de-gassed as N2. So realistically it is a re-circulating Winogradsky column, the problem comes in that it has to be cycled, to stratify the bacteria, and it is quite tricky to get the right water flow.
The other possibility is the freshwater plenum. The idea of encouraging this bacterial community and applying to planted freshwater aquaria the reefkeepers' "Jaubert" system, with a low-oxygen area in the lower substrate (the "plenum") where de-nitrification proceeds, usually it uses a chemically inert ceramic-glass-foam material "Cell-Pore." Cell-Pore has very small interconnected pores of 50-400 microns. If you have a deep, relatively un-disturbed substrate you will have a de-facto plenum anyway.

And I see there is now another comment by "Brenmuk",  I think all the points you make are probably correct.

cheers Darrel


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## foxfish (13 Dec 2009)

Hey Darrel, if you look at my picture you can see that in fact it is of a freshwater plenum!
There is a PVC perforated plate suspended on top of sintered glass, the tubes are 25mm high. 
The PVC is covered in 150mm of inert gravel = freshwater plenum.
However i dont think it actually worked that well! The set up was linked to a 300lt discus tank but the No3 was always above 10 ppm!
Plenums definitely work in salt water though as I had the same design on a reef tank that operated with less than 2ppm.
Fox.


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## dw1305 (14 Dec 2009)

Hi all,
Foxfish apologies, I understand the theory of their application, but I'm not very up on the practicalities of either design.
I've never tried to use a plenum, although some proportion of the substrate in my tanks is operating as one. I think that in the planted tank with a relatively undisturbed substrate it's a "win win" situation with regard to nitrate levels (should you wish to limit these) in the aquarium water. If you have a lot of root (and rhizosphere) the plants actively up-take any available oxygen from a largely aerobic substrate, and if you have fewer roots (or a less permeable substrate) the larger anaerobic areas acts as a plenum and de-gas the nitrate to N2. I like a few MTS and Asellus etc in my tanks partially to have more natural processes occurring in the substrate, in the same way a Marine keeper would use living sand/rock.

The only problems would occur if you got very anaerobic substrate and had a large build up of sulphide producing bacteria. When we had the Winogradsky column in the lab. we moved it (to take to a science fair) and when it was disturbed it released so much hydrogen sulphide we had to evacuate the building.

cheers Darrel


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## foxfish (1 Nov 2010)

Can I regenerate this topic - I am still wondering if introducing C02 though a under gravel plate would work?
It just seems that a even upward flow of C02 enriched water would offer just about the best distribution of the gas?


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## foxfish (26 Apr 2011)

I am still interested in other members opinions about this subject...

I have just re read the post but I think the point I am trying to make has not been fully understood!

I am not interested in the biological effects or benifits of an under gravel filter - I am interested about the even distribution of C02!

I still think if one fitted an inline atomizer & sent the C02 enriched water down an UGF uplift it would appear in a very even distribution at the bottom of the tank & slowly rise to the surface? 
Surly this would be an efficient way to get C02 into every possible area of the tank?


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## ceg4048 (26 Apr 2011)

Hi mate,
             This has been tried with mixed results. See Hoppy's Planted Tank thread RFUG - A Blast from the Past - DIY

Cheers,


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## foxfish (26 Apr 2011)

Thanks - well after reading through Hoppys thread I think I will be giving the idea a go..
However I feel I can build a far more efficient system than Hoppy experimental effort (no disrespect meant as his info makes very helpful reading).


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## Mat H (5 Aug 2021)

Just correcting Clive's link in case someone ends up in this thread like I did and wonders what the link thread said!









						RFUG - A Blast from the Past - DIY
					

One problem I never seem to get away from is finding a way to get good CO2 concentration all over the tank.  I can and do set up a Koralia powerhead to give me a circular water circulation all around the tank, but as the plants grow, that circulation gets slower and slower.  So, I am tired of...




					www.plantedtank.net


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## Ady34 (7 Aug 2021)

Hi, 
I would think the major failing of ug filters in planted tanks would be getting and even flow through the substrate. I guess water will flow via the easiest route so once hardscape and further down the line, root growth have taken their place,  then water flow will be compromised in large areas of the substrate and also limited within the aquarium itself.
Perhaps a Dutch style aquarium would be best suited, that is until the root systems overdevelop and prevent efficiency? 
This is only speculation of course, and if the substrate itself is not to be used as the primary filtration method (external canister to drive the water flow) then it’s worth a go as co2 and o2 to the roots can only be a good thing…….


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## foxfish (7 Aug 2021)

I still think it is worth far more investigation,  a simple fine mesh incorporated on top of the filter might help although I do agree the filter is quite likely to clog up in a year or so, however the potential to offer such an even distribution over the whole tank could solve a lot of common problems.


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## X3NiTH (7 Aug 2021)

This is where making GoreTex/PerTex bladders made up kind of like how ice cube bags work with compartmentalisation to prevent one giant balloon, could allow slow diffusion of gasses through. I still haven’t tried this even though I mentioned it many many moons ago. I suppose you could construct the UGF in a way that you could slip the bags out if they needed replacing or washing. The finer the pore of the fabric would be better allowing for a smaller bubble size and thus quicker dissolution of co2 up through the substrate.


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## akwarium (8 Aug 2021)

- roots often grow into the UGF making it hard to (re)move a plant.  
+ it is a cheap, good looking and well working filter method. (no hoses, no leaking, no noise, no big black box inside your tank)

all other aspects are trivial imo.  soil biology, distribution of nutrients and co2... it may be different but it is not better or worse.  Just another route to the same destination.

I use them quite often on small projects, mostly because they can be placed everywhere.


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## foxfish (8 Aug 2021)

You use them to flow C02 enriched water through?


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## X3NiTH (8 Aug 2021)

No, direct gas injection under low pressure, the GoreTex/PerTex pore size is small enough to be a barrier for water so when you pass co2 gas into the bladder it can diffuse through these pores and because you can make the bag fit the entire area on the bottom plenum/ugf the corresponding substrate area above the bladder will saturate with co2 as it diffuses/bubbles through. Does that make sense?


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## akwarium (8 Aug 2021)

foxfish said:


> You use them to flow C02 enriched water through?



 In most of my small projects I don't add co2.  
if I do the effect is not different then in any other setup with a different filter method.

Water movement in an aquarium is strongest where it is actually moved. so near the pump.  If you have a spraybar more water will go trough the first hole then to last one. If you have a UGF more water will flow to the substrate near the pipe where water is blown in or sucked out, then on the other side of the tank.  

if you want to distribute CO2 and nutrients better, move more water.


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## foxfish (9 Aug 2021)

Well that would depend on the style of under gravel filter?
In my case a suspended, perforated  plate allows for more even distribution If the return water is jetted under the plate.
I have also used a matrix of perforated pipe under a perforated plate and fed the water into the pipe… a sort of mutable spray bars under the plate.
Next up is the type of substrate as fine sand may well pressurise the under gavel stytem and cause a build up of C02 where as an open media like cat litter will allow a more even flow.
I have experimented with both those styles and they both worked very well!
The ideal is to introduce an even (ish) distribution on gas up through the media and then circulate the main tank volume.
I found a few picture but not really the ones I wanted as most were taken many years ago!


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