Plenums/denitrators

[Ash]

I was reading on www.garf.org about plenums, most specifically this...

Most of the systems at GARF (all of the Bullet Proof Reefs ) use plenums and have been very successful.  Plenums create a dead water space area under the substrate.  This dead water space, along with the substrate, help in the denitrification process which lower the nitrates

Now bacially the lack of oxygen in this dead spot lets anaerobic bacteria break down nitrate into nitrogen & oxygen gases (same as a coil denitrator, refer here--> http://archimedes.galilei.com/raiar/denitrif.html )

Can you get a functional anerobic bacteria colony in a freshwater aquarium?

The reason I'm asking is my upstairs 4x2x2 seems to NEVER get a nitrate reading over 10ppm, no matter how long I leave it without a water change. This pretty much goes against logical thinking. I have a latex background that isn't sealed into place & hence has quite a lot of water trapped behind it (with no circulation it's quite yucky I might add) - it is working as a plenum to reduce my nitrates?

Filtration is one a DIY wet/dry trickle filter with a King-3 return pump.

Any input appreciated.

Ash

[foai]

Anthony from Auburn Aquariums gave a very fascinating talk on the use of plenums in Tanganyikan tanks at a Cichlid Society meeting about 6 months ago.

He said that providing the plenum was correctly set up using the different diameter substrates and with sufficient space in the void at the bottom of the tank a plenum will work perfectly well. I think (but am not sure) he said that to test its efficacy he did not do a dedicated water change for about a year and just topped up the water that had evaporated. In all that time his tropheus continued to thrive.

I was very impressed by the talk and the results he got.

Provided you set it up correctly it looks like it is a reall winner.

David

[Ducksta]

Plenum's absolutely work in freshwater There are many designs about too.

I am considering just filling an ice cream bucket with gravel, putting fly mesh or the lid with lots of holes on top, and dropping them in tanks. However, what i think I might do, and what I get around to doing are generally way off course of each other.

[Ash]

so I'm not going nuts? could the background provide enough of a limit to water flow to make it work though? cause my understanding is it's the lack of oxygen in the water that makes a plenum work?

theres a stack of basically stagnate water (maybe 50l?) behind the 3d latex background, I'm scared of upsetting it incase it poisons the fish.

[r.d.m]

if anerobic bacteria are present youll generally get a hydrogen sulfide(rotten egg) smell, you nearly always get it in deep gravel beds when you disturb them , idont think itll harm fish because the smell comes straight out the top of the water and is generally gone in a few seconds, however only having 2 tanks i prefer water changes. i think the system was developed primarily for hard and soft coral keeping, their being used to nutrient poor water and little tolerance for nitrates and the resulting algae growth, dupla put out a system that ran on methanol! i personally think it has little value in fresh water unless you have a multiple set up running on a single sump where it could be effectively incorporated, but if youve got one for free good luck to you!

[Brett4Perth]

My understanding of plenums is that they have to produce hypoxic zones to work effectively. Hypoxic zones are areas of low oxygen. If there is no oxygen, anoxic, then it doesn't work as well. Consequently depth of substrate, size of plenum, particle size of substrate and water flow are all important.

In a freshwater tank it is much easier just to have a few floating plants that will remove the nitrates from the water.

This doesn't remove the requirement for water changes. Lots of other chemicals are diluted by water changes as well as nitrates.

Merry Xmas

Brett

[Daniel]

This is a very interesting topic, as I have always doubted the need for extensive water changes in a lightly stocked tank or a basic planted tank. My tanks have never had a nitrate reading over 5ppm, even if I don't do water changes for several months. The aenerobic conditions that are necessary for converting nitrate to nitrogen gas are present in a lot of filters, particularly cannister type filters, and even more so when cannister filters are heavily filled with media or a little blocked.

I know of a few people who keep tropicals in non planted tanks and have never ever done a water change, just everaporation top ups and filter cleaning, and have no fatalities. The only explanation for this is that nitrates are being converted to nitrogen. I also believe that the finer your gravel the more chance of aenerobic conditions in under the gravel bed also. If fish life and plant life can survive and even thrive in a small enclosed pond, then I see no reason why this can't be done in an aquarium.

I would be interested to know more about the conditions in the tank that Anthony has, ie filtration type and servicing schedule, fish load etc.

[CThompson]

What Brett4Perth is saying about depth

of substrate, size of plenum, particle size of substrate and water flow are all important

is spot on. A plenum is an area where there is no oxygen, but to get the chemical movement that facilitates the way a plenum works, requires careful setting up of the substrate.

Ash, the area behind the back of your background will NOT be working as a plenum; all you have here is a pollution source for your tank with no upsides.

Daniel

I know of a few people who keep tropicals in non planted tanks and have never ever done a water change, just everaporation top ups and filter cleaning, and have no fatalities.

These are also the people that will eventually say “gosh I don’t know what happened; the fish were all okay yesterday”. Or take newly purchased fish back to the shop and say “there was something wrong with your fish, as soon as I put them in my tank they all died”.

If you want to find out more about a plenum, Google it, there is sure to be heeps on the internet.

There is a simple alternative to a plenum. There is a product called Denitrate, produced by SeaChem. It is like efisubstrat, but the size of kitty litter. You pack this stuff into an enclosed container (i.e. canister filter), and limit the amount of water that passes through the enclosure.

I have made my own canister filter by using a PVC pipe, with both ends closed off with bulk-head (tank outlet) fittings plumbed to both ends. This has water gong through it that is by-passed off my return from the main mechanical filter on the tank (which means the return water will already be reduced in oxygen). With the aid of and inline tap, I regulate how much water going through this anaerobic area. My Tropheus tank has never had a nitrate reading.

Incidentally, this is no excuse for not doing water changes. Once nitrates are broken down may be the end of the nitrogen cycle. However, there are more things in the water that are exported with a water change than the end product of nitrification.

By all means set up a plenum (provided you do it correctly, as if you don’t you are wasting your time), but think about using Denitrate as it is much easier to set up, requiring less knowledge than a plenum, and is more robust. It also doesn’t take up room in the tank, and won’t be affected by digging cichlids.

Craig

[Gav]

These are also the people that will eventually say “gosh I don’t know what happened; the fish were all okay yesterday”. Or take newly purchased fish back to the shop and say “there was something wrong with your fish, as soon as I put them in my tank they all died”.

sorry craig, I will disagree with you here. I have managed to do a similar thing for a community tank and only did evaporation top ups for three or so years without a problem at all. i had two canister filters running and they got yearly cleans. wasn't at a heavy stocking level, but certainly not light. there was the odd plant in the tank, but certainly not a planted tank. I wouldn't like to even consider it with cichlids, but on sensibly fed community fish that arent particularly messy it is entirely possible

[Daniel]

Craig

"These are also the people that will eventually say “gosh I don’t know what happened; the fish were all okay yesterday”. Or take newly purchased fish back to the shop and say “there was something wrong with your fish, as soon as I put them in my tank they all died"

I will also strongly disagree with you Craig. Those people who never do water changes do not have fatalities ever. I dont think you realise that 'no water change' means no water change, nil, zero, none are ever needed. From the examples I have seen and from my own experience, it comes down to the fish load on your filtration system, together with water volume, and perhaps gravel grain size. I have absolutely no doubt that alot of filters will actually denitrify, it is obvious by the results of nitrate readings. How else can you explain continuous 0 ppm (or near zero) nitrate readings continuously. In fact I rarely do nitrate readings as the results are always the same. And I have used 3 different test kits, and the same results. Now, I am not for one minute suggesting it is possible to do this with a tank full of hungry mbuna's or with large American cichlids. It all comes down to the fish load, water volume and then other facters such as

filtration system, gravel bed etc.

I suspect there are quite few fish keepers who take the 'herd' mentality and follow eachother into doing excessive water changes. On the other hand, those with higher fish load have no choice.

[Ash]

Ash, the area behind the back of your background will NOT be working as a plenum; all you have here is a pollution source for your tank with no upsides.

I was only looking for some reason for the constant unwavering 10ppm reading (two diff kits used) & thought maybe the heap of still water might have been an explanation, I only used the term "plenum" to get people on the right page.

This tank I'm talking about it a 4x2x2 with a few medium CA/SA cichlids & a gibbi so they have a fair whack of mess/bioload, so the non rising nitrate reading is a bit confusing is all & I was curious as to why.

[Daniel]

Ash,

If your getting 10ppm and have tried two test kits for the same result, you really have to think twice about doing water changes unless this figure rises. It would appear your set up is able to cope with the current bio load, so only evaporation top ups are necessary.

[Brett4Perth]

If your getting 10ppm and have tried two test kits for the same result, you really have to think twice about doing water changes unless this figure rises. It would appear your set up is able to cope with the current bio load, so only evaporation top ups are necessary

I am afraid I disagree with this Daniel. Nitrate is not the only waste product and most of them can't be measured easily and are probably not removed by filters. The only way to dilute these chemical is with water changes. They may not necessarily be lethal, but your fish will be healthier.

Cheers

Brett

[Daniel]

So Brett at 10ppm Nitrate concentration, what is a 20% water change going to do, take it down to 8ppm ! Is there much difference ? With evaporation top ups, they are getting new fresh water, albeit in small doses. Obviously one could also measure nitrite and ammonia, but this is not really necessary in an established tank as most set ups can easily handle those conversion stages. Think about it, do you measure nitrate content an a regular basis and realise how necessary that next water change is ? Or do you just do it out of habit ?

This happens in nature in small ponds. These ponds are subject to evaporation and rainfall. No water changes. There is no reason this can't happen in an aquarium, it comes down to 'capacity' of your set up.

[Brett4Perth]

I think you have missed my point. I am not suggesting that nitrates of 10ppm are a concern and I would hope that all tanks have zero nitrites and ammonia.

What I am saying is that there are other chemicals, nothing to do with the nitrogen cycle, which can and do influence fish health. These include but are not restricted to other metabolic wastes and hormones secreted by both plants and fish. These chemicals can retard fish growth and interfere with health. The only way to reduce these chemicals is by dilution ie. water changes. They are not removed by "top-ups".

These chemicals may not be lethal in many instances, but there is a difference between living and optimal health. Water changes improve the health of your fish irrespective of nitrate levels.

I am sure you would agree that in nature there are very few instances of fish persisting at densities even close to those we routinely maintain in aquariums. There is just not enough food to maintain these stocking levels. You may see such densities occasionally, but usually this is a transient state such as a pool drying up. Hardly optimal conditions for the fish.

You are right that at some point the system must be sustainable but I doubt any aquarists actually stock their tanks at these levels. I have a 4 ft tank (200l) which contains only a pair of blue rams. Perhaps this is sustainable with just "top-ups" but is anyone prepared to accept such low stocking levels ??

Cheers

Brett

[Daniel]

Brett,

Whilst I agee with some of your points, I am not so sure about paragraph 2, as in nature small ponds do not get water changes. They simply get evaporation and new rainfall. They are fully sustainable without any electrical filtration methods that we have the privelage of using in our tanks.

By the way, you should know that any aquarium receiving partial water changes over many years, that aquaurium will still have a small % of the original water when the aquarium was first set up many years earlier. One will never get a full dilution unless a 100% water change is done.

And the suggestion of your 4ft tank with Rams being self sustainable, I have no doubt that a significantly higher fish load than this will could be self sustainable.

Just last night I spoke to a interstate friend with 20+ years experience, and he has no doubt that most filters denitrify and he has kept some self sustainable aquariums over the years.

[YeW]

Guys -

If you have deep layers of gravel in an aquarium my belief is that you get some of this "plenum" effect without the plenum. In cichlid tanks we tend to have thin gravel layers which dont provide the same effect. This in effect can "buffer" some aquariums against the harsh realities of bad (or no) water change regimes... but agree with Brett and Craig that such systems are often unstable and are prone to problems in the longer term.

I have found that even under 6" of 5mm gravel there is enough oxygen to support growth of some brown algae species... so you need to do quite a bit to get true anoxic (or hypoxic) zones in gravel beds.

In addition - I've used denitrating coils - which I believe are better than plenums (due to the increased flow rate).

HTH -