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Building an anoxic system     Building an anoxic filtration system

Building an anoxic filtration system
The anoxic system is inexpensive to build and is adaptable. It will fit anywhere because it can be built to fit whatever space is available. The only constraint is that there should be about one basket per adult fish but, even in this, there is flexibility.  If ever it is found that more baskets are needed for an increased stocking level, and space is limited, an extra layer can be stacked on top of the bottom layer as long as they are spaced so that the bottom of one basket doesn’t rest directly on top of the one below. This will ensure that water can still flow past all surfaces of all baskets.

Anoxic filtration - spaces around baskets clrday

Small spacers allows water to flow past all surfaces of the baskets

A basket where water is able to flow past all six surfaces, (the four sides plus the top and bottom), will be able to remove ammonia more efficiently than a basket where water can’t flow past one or more of the surfaces. The gap doesn’t have to be great and it can be seen from the picture below of an anoxic pond that, in order to minimise the footprint of the pond, the baskets have been placed with their tops actually touching.

However, since typical baskets taper toward their bottoms, even if the tops are touching, there will still be adequate spaces between them for water to freely circulate around all sides. If small spacers are placed underneath the baskets, they will allow water to circulate underneath them too.

Stacked baskets can reduce the anoxic pond footprint
Anoxic ponds can be of any shape or size to suit available space and the depth of 600 mm (24 inches) is only a suggestion that will suit a wide range of common aquatic plants. The depth isn’t important to how efficiently the baskets will function; they will work at any depth that is convenient to the hobbyist from being just covered by water to a depth of one metre or more.

Deep anoxic ponds will allow the baskets in it to be stacked in order to get the maximum number of baskets into any given surface footprint. However, if a deep anoxic pond is considered, thought should also be given to the ease of future maintenance.

As a guide, there should be about one basket per adult fish for the system to provide stand-alone anoxic filtration, just putting one or two biocenosis baskets somewhere in the pond may not have any significant effect.

As a guide, there should be about one basket per adult fish for the system to provide stand-alone anoxic filtration, just putting one or two biocenosis baskets somewhere in the pond may not have any significant effect. The optimum size of the biocenosis basket is 30 cm x 30 cm x 20 cm. It’s possible to use smaller baskets but they will have less room inside them for anoxic conditions to develop so they won’t be as effective as the optimum size.

Larger baskets would allow for a larger anoxic zone to develop but will become very heavy once they have been submerged. A point that should be remembered is that each basket may have to be lifted out to allow for occasional maintenance of the basket itself or cleaning any leaves that may blow into the anoxic pond or sediment that may accumulate in it. A leaf net over the anoxic pond will reduce maintenance.

Anoxic pond without plants
A typical anoxic pond without plants

This example of a typical anoxic pond contains 22 baskets that are 30 cm x 30 cm square by 20 cm deep and which are contained in a pond that is 1.8 m long by 1.2 m wide. The water depth over each basket is around 40 cm.

These dimensions aren’t crucial, they are only for guidance. This particular pond is just an example of a compact way to house 22 baskets.

Any other convenient shape would be just as suitable as long as all the baskets are completely submerged and that water can flow gently through the pond without disturbing the contents of the baskets.

Avoiding settlement in the anoxic pond
Whichever filtration method is used, conventional or anoxic filtration, a turnover rate of once per hour or greater is recommended. The faster ammonia is removed from where the koi are excreting it and presented to the biofilter the better because the overall effect will be to reduce background levels of ammonia in the main pond.

Since nitrifying bugs rapidly colonise any wet surface where there is also a source of ammonia, every surface in a koi pond will provide a home for the ammonia bug (nitrosomonas). Their waste product, nitrite, will then provide a nutrient source for the nitrite bug (nitrobacter) which will also colonise every wet surface in the pond. The result is that there will always be a degree of the nitrogen cycle conversion of ammonia to nitrate in koi ponds.

Anoxic filtration - sieve 1With conventional filtration, this isn’t a problem because the biofilter will also be converting ammonia to nitrate so any similar action taking place in the main pond will assist the biofilter not oppose it.

Where anoxic filtration is employed, the ammonia is removed from the system without leaving nitrate as the end product. Therefore a quicker rate of removal of ammonia from the main pond and the consequential reduction of background ammonia in it is not only of benefit to the koi but it also reduces, as far as is possible, the nitrogen cycle taking place in the main pond. This allows the majority of ammonia to be removed and dealt with in the anoxic pond without nitrate being produced as an end product.

Accordingly, the recommended flow rate through an anoxic pond is as fast as is possible without disturbing the contents of the biocenosis baskets. Even with a relatively high pond turnover rate, the rate at which water flows through the anoxic pond will be far slower than the rate at which it passed through the pipework leading to it. Just as high winds will be able to blow over dustbins but a gentle breeze will only be able to blow paper litter around, water travelling at higher speeds is able to carry more silt, and heavier silt, than water travelling at slower speeds. The slower rate of flow through the anoxic pond than in the pipework leading to it inevitably will lead to it being unable to carry so much suspended silt so there will be some settlement of the suspended particles that the water had been carrying. Provided any settled silt isn’t allowed to build to a depth of more than a few millimetres and it doesn’t contain any organic debris such as fallen leaves, it will become inert and will not lead to a subsequent deterioration in water parameters.
Rotary drum filter
One recommended addition to an anoxic system is a pre filter. Installing a good pre filter before the anoxic pond will remove most of the suspended particles. This in turn will substantially reduce the amount of silt that will subsequently accumulate in the anoxic pond although, even with a pre filter, there still will be some settlement. Any good pre filter, including brushes or a settlement chamber, can be used before the anoxic pond.

Trials by Kevin Novak and the users of anoxic filtration in the UK have shown that two types, a sieve or a rotary drum filter are particularly effective at removing suspended particles which will reduce the frequency at which the anoxic pond will have to be cleaned. Another biological filter that has been used by another UK user to good effect as a prefilter is a pump fed bead filter.

Plumbing is simple for either the original pump fed system, as is shown in figure 8, or the gravity system in figure 9. Other pre filter arrangements such as rotary drum filters, brush chambers or settlement chambers can be used especially if they already exist and the anoxic filter system is being used as a replacement for an existing conventional filter system. The plumbing shown in the diagrams will remain substantially the same if other types of pre filters are used instead.

The sieve and the associated plumbing in figure 8 can be replaced by a rotary drum filter or any suitable pre filter or settlement chamber that is compatible with being pump fed and which can then flow by gravity via the anoxic pond back to the main pond.

The sieve in figure 9 can be replaced by any pre filter that is compatible with being gravity fed and with an output that can be pumped to the anoxic pond.

Anoxic filtration - pump fed sieve

Anoxic filtration - gravity sieve

Removing the residual settlement that does collect in the anoxic pond

Earlier diagrams have deliberately been kept as simple as possible and have shown only the essential principles of how the anoxic filter system works and integrates with standard design koi ponds and equipment.

Interrupting the anoxic environment will only be a significant factor if the biocenosis baskets are left to drain for too long during cleaning or transferring to another pond …..
.…. I don't see a problem arising from cleaning the anoxic pond provided it isn't done every week.

The diagrams in this section show the extra detail of drains and associated waste pipework which can be periodically used to flush away any remaining settlement that does collect. A thin layer of fine silt with no accumulated fallen leaves or other organic matter will become inert so, with a pre filter, the anoxic pond won't need cleaning too often.

When cleaning the anoxic pond, a point to bear in mind but which shouldn’t be exaggerated is that, if the anoxic pond is completely drained or if a biocenosis basket is removed from water, the oxygen depleted water in zone C obviously will drain out. The water in the basket won't immediately drain completely; some oxygen depleted water will remain inside and will only drain slowly if the basket is left out of water for too long. When the pond is refilled or the basket is resubmerged, the anoxic environment will be removed as aerated water floods back in.

The anoxic environment will soon be restored but, depending on how much water had drained out and therefore how much aerated water goes back in, the interruption will either be a short blip or, in the case of a complete dry out, it may take much longer. Interrupting the anoxic environment will only be a significant factor if the baskets are left to drain for too long during cleaning or transferring to another pond so that all the oxygen depleted water drains out. I don't see a problem arising from cleaning the anoxic pond provided it isn't done every week.

As described earlier, in order to minimise the pond footprint, baskets may be positioned close together and possibly even with the tops of tapered baskets touching. However, if space permits, it might be prudent to have them slightly separated so that a hose may be used while the pond is draining to help wash the silt towards the drain in order to speed up the process.

Earlier diagrams have been deliberately kept as simple as possible and have shown only the essential principles of how an anoxic filter system works and integrates with standard design koi ponds. The diagrams in this section show the extra detail of drains and associated waste pipework.....

If a hose is used, care should be taken not to run it too fast or direct the water directly onto the sides of the baskets in order not to disturb their contents. To help achieve this, a length of pipe could be fitted to the hose to help direct water at the floor in between the baskets.

Any pipe will do for this purpose but 15 mm (½”) copper pipe is a suitable size and is conveniently light and strong. Despite sensible warnings about not using copper pipe in koi pond plumbing systems, a metre or so of copper pipe used in this way will not introduce copper into the water.

When cleaning the anoxic pond, it isn’t necessary to drain it completely, it’s only necessary to open the drain valve for as long as it takes to clear the floor but, if a great deal of water is lost in the process, the water won’t have been wasted, it will count toward that week’s quota of water changes.

The pump fed anoxic filter system
Figure 10 shows the waste valves and associated plumbing for a pump fed anoxic filter system with a prefilter. The floor of the anoxic pond is shown slightly sloped towards the drain because this will help remove any settled silt when the pond is being flushed.

Anoxic filtration - pump fed sieve + waste

In normal operation valve 1 is closed and is only opened to flush the anoxic pond. The waste pipework to the anoxic pond should be at least 2”. Good quality 4” valves are expensive but, if the budget allows, clearing the floor will be far more effective with the larger size since the increased flow to waste will create a greater “suction”.

Unless the prefilter requires a permanent connection to waste, valve 2 is normally closed and only opened to flush it. The waste plumbing to the prefilter should be sized as per manufacturers’ specifications for the type that is being used.

The gravity fed anoxic filter system
Figure 11 shows the waste valves and associated plumbing for a gravity fed system with a prefilter.

Anoxic filtration - gravity sieve + waste

As described above, the floor of the anoxic pond is slightly sloped towards the drain to assist flushing away silt. The waste plumbing and operation of the two valves is also the same as above with valve 1 normally closed except when flushing silt from the anoxic pond and valve 2 normally closed and only opened to flush the prefilter unless it requires a permanent connection to waste.

It’s advisable with conventional gravity fed systems that a valve should be included that will allow the bottom drain pipework to be regularly flushed to waste in order to flush out any silt that has settled in it. Valve 3 has been included for this purpose and, as is usual with these valves in a conventional system, it is kept closed during normal operation and opened, on a regular basis, as part of normal pond maintenance regardless of whether silt can be seen in the anoxic pond.

The airlift fed anoxic filter system
Figure 12 shows the waste valves and associated plumbing for an airlift fed system.

Anoxic filtration - airlift design + waste

As with the above options the floor of the anoxic pond is slightly sloped towards the drain to assist flushing away silt and valve 1 is normally closed except when flushing silt from the anoxic pond. In order to avoid the build up of settled silt in the bottom drain pipework to the airlift, valve 2 has been included and is opened, on a regular basis, as part of normal pond maintenance regardless of whether on not silt can be seen in the anoxic pond.



A novel approach to building an anoxic pond

This step-by-step build of an anoxic pond was documented and kindly supplied by Brian Woodcock.

Anoxic build 1
Paving slabs were cut with a 4" disc cutter and laid as a base.

Anoxic build 2
Construction of the wooden framework for the anoxic pond is 4 x 2 inch timber.

Anoxic build 3
One side and the two ends of the framework are quickly finished and in position.

Anoxic build 4
The framework is completed. Note the supports for the raised floor which allow room for the waste pipe.

Anoxic build 5
The sides and the floor of the anoxic pond are made of ½ inch marine ply.

Anoxic build 6
The total cost so far for the completed framework and ply lining is (GB) £180 [2014 prices].

Anoxic build 7
The diffuser that will spread the flow into the anoxic pond is completed.

Anoxic build 8
The diffuser is fitted.  When the liner is installed it will be turned through 45°.

Anoxic build 9
Under the floor of the pond, a ball valve was used on the waste line as they are more reliable than a cheaper slide valve.

Anoxic build 10
A boxweld liner (a heat welded prefabricated 0.75 mm EPDM or Butyl liner) provides the waterproof finish for the anoxic pond.

Anoxic build 11
With the liner in place the return inlet, (feed from the pond), was fitted through the ply sides and sealed to the liner.

Anoxic build 12
After fitting the return inlet, the fitting for the drain is fitted and sealed in the same way.

Anoxic build 13
The diffuser is turned through 45° to spread the flow across the whole of one end and provide a more diffuse flow.

Anoxic build 14
The anoxic pond copings match and complement the existing pond copings so as not spoil the look of the main pond.

Anoxic build 15
The water chute that will return water back to the main pond also matches the copings.

Anoxic build 16
Seven biocenosis baskets down nineteen to go.  Note that lengths of pipe are used to space baskets are from the bottom.

Anoxic build 17
Three hours later all the biocenosis baskets are filled.  The layer of stones prevents water flow disturbing the cat litter.

Anoxic build 18
A major goal was that it shouldn’t look like an add-on.  Plants will be added next to make it look less stark.

Anoxic build 19
Before the anoxic filter was started the nitrate level was over 80 mg/L.  Just ten days later it was already starting to fall.

Anoxic build 20
Before the anoxic filter was started the nitrate level was over 80 mg/L.  Just ten days later it was already starting to fall.

Anoxic build 21
After two months nitrate levelled at 30 mg/L despite feeding 300 g per day to 21 koi and a bead filter producing nitrate.

Anoxic build 22
A lily that was very small when bought is already producing its first bud.

Anoxic build 23
A few hours later the first water lily opens and compliments the luxuriant plant growth.

Anoxic build 2402
To clean, after emptying, a hose is used to rinse mulm towards the drain. Total time from start to finish - 20 minutes.




Anoxic filtration in the UK

The anoxic filtration system was design and developed by Dr Kevin Novak PhD and in the US, where Kevin lives, all cat litters are pretty much the same. In the UK there are many different kinds of cat litter type products and manufacturers sometimes change brands and ingredients so those intending to use anoxic filtration will have to look around to see what’s available in their area.

Cat litter 01
Cat litter 02
Correct types of cat litter

Below are some commonly available brands that have retained their structure in tests and which have been used successfully but you should check that the composition hasn’t changed before buying large quantities.

Obviously don’t try to use any cat litter that is made from recycled paper or wood pulp or which promises to naturally decompose in an environmentally friendly way when disposed of after the cat has used it.

The panel on the left shows the type of granular clay that you should be looking for. The colour isn’t important and it need not be little round granules but it mustn’t be so fine that it packs down like sand. Biocenosis baskets need to have open pathways in between the granules in order to allow the ammonia to be drawn into the centre. The panel on the right shows types of cat litter that should not be used.

The cat litter should be thoroughly rinsed in order to remove any dust but litter that contains excessive amounts of dust won’t be suitable since there may be very little of it left when all the dust has been rinsed away.

Clumping cat litter03
Clumping cat litter

Recycled paper pellets02
Recycled paper pellets

Odour control litter02
Odour control pearls

It’s preferable not to use scented cat litter but if you can’t find a suitable unscented one then a scented litter can be used as long as it doesn’t contain either scented or odour control pearls and provided the perfume is thoroughly rinsed away. Products described as “Clumping Cat Litter” are invariably useless because, when they get wet, the granules clump together. This is perfect when cat litter is used for its original purpose but if it is permanently submerged it will just become a soggy mush.

Unless you have a cat or know someone who will appreciate an opened bag of litter that turned out to be ideal for a cat to mess in but no good for you, instead of buying a bag that looks promising, try to obtain a few grains from it, take them home and put them in a glass of water overnight. Squeeze them in the morning and if they go to mush, they're no good. If they seem ok, leave them for several days and try again.

When you find a product that retains its granular structure after being submerged for several days, make up a test basket without the laterite and submerge it for at least a week and then take it out and delve into the cat litter to make sure it still retains its granular structure. If it does then you have found a suitable product so you can add the laterite to that one and obtain sufficient cat litter of that brand to make all your baskets, complete with laterite, and use them straight away in full confidence.




Brands of cat litter on sale in the UK that have been reported as being suitable

Before buying large quantities of cat litter, even the brands shown below which have been tested and found to be successful, it’s advisable to first check its suitability in order to ensure that the manufacturers haven’t changed the type or the grade of clay used. This is easy to do using a small amount and the method described above.


Steetley Sophisticat Original
Steetley Sophisticat Original

Tesco odour control06
Tesco odour control cat litter

Tesco cat litter
Tesco low dust cat litter

Tesco premium cat litter
Tesco Premium cat litter

Alternatives to cat litter
There are places where a suitable type of cat litter simply isn’t available. Lio Fornellino (Dr. Franco), from Italy, was keen to test the performance of the anoxic filtration system. He searched throughout Italy for a suitable product but was unable to find one.

Lio teeshirtHe asked for help in locating an alternative on an anoxic filtration forum. One UK user of the forum bought a large consignment, arranged to take it to Dover (UK) and Lio drove from Italy to collect it. He tested the system under different scenarios and was so impressed with the results that, before his untimely passing in October 2012, he went on to promote anoxic system throughout Italy via his blog.

Since then, another possible source of the correct type of clay has been brought to my attention. It is used as an addition to bonsai soil and is slightly more expensive than cat litter although current UK prices for a 25 litre bag are competitive when compared to 2½ times the cost of 10 litres of a suitable cat litter. In the UK it is marketed as Biosorb®, Ultrazorb® or Danish Pink® and anyone having trouble locating a suitable product in their area might consider looking at what is available via bonsai equipment selling sites on the Internet.

Additionally, some people have found that a product called “Oildri®” or plain “Fuller's Earth” are consistently suitable types of media.




Laterite 135 230LateriteLaterite is a type of clay that is rich in iron and which naturally forms in hot, wet tropical areas.  In addition to its iron content, it also attracts and holds phosphates which it allows plant root hairs to uptake as a nutrient so it is mined for use as a plant fertiliser especially for aquatic environments.

Laterite can be found on the Internet for less than £20 / US $30 / €27 including postage for the 1.6 kg size. [January 2015 prices and currency conversion].

This size will be sufficient for between four and eight baskets using about 200 gm to 400 gm per basket depending on the basket size. As a guide, for effective stand-alone anoxic filtration you will need about one basket per adult fish.

Update on laterite availability in the UK
At the time of writing this update (April 2015) supplies of laterite in the UK have suddenly become difficult to obtain. Supplies from other sources outside the UK are still available but this will incur higher postage costs. At the moment it isn’t clear whether this will be a temporary or a permanent situation or whether this interruption to supplies is also affecting other areas so a couple substitutes have been suggested by Kevin Novak, the designer of the system.

JBL Aquabasis bag 22502
JBL Aquabasis clay 150JBL Aquabasis plus
This aquatic plant fertiliser contains all essential nutrients such as iron, minerals and trace elements.  The clay pellets act as a nutrient store, absorbing excess nutrients and releasing them again as required.

It has been successfully used as a substitute for laterite and is currently on sale from www.amazon.co.uk/ at £16.69 including postage to UK addresses.

Ironite 22502
Ironite 1-0-1
Although Ironite 1-0-1 isn’t currently available from UK suppliers it has been included in this update in case potential anoxic filtration users outside the UK are also having trouble obtaining laterite but find Ironite 1-0-1 easier to obtain from their local suppliers.  It can be ordered on-line from suppliers outside the UK.

There is a full range of Ironite fertiliser products made by Pennington Fertilizer Inc® so anyone intending to use this as a substitute for laterite should ensure that they obtain Ironite 1-0-1 rather than another fertiliser with a similar name from the range.

Ironite 1-0-1 is primarily marketed as a garden fertiliser but it has been tested by Kevin and found to be a safe and suitable substitute for laterite.

Ironite stocking pouches (cutting)Ironite stocking pouchesTo prevent it clouding the pond water for a day or two it must be contained in a small pouch made from a nylon stocking as shown.

The completed pouches should only contain about half the amount of laterite that would have been use, (i.e. 100 grams for small baskets and 200 grams for larger ones), and should be buried in the centres of the baskets.



Plants 204

There are no specific plants to use. As long as they will grow during the summer season rather than be a static constant size and will also not develop a root system that will grow so large that they will burst the basket you can choose plants that you like and they will be fine

Plants 403

Plants 103

Plants 504

Plants 303



Mugshot 17502Author bio
In 1984 Syd Mitchell started a swimming pool company with the main aim of designing and developing filtration and water chemistry control equipment for the leisure industry.

Since retiring, Syd spends his free time studying koi physiology and how it is affected by water parameters.  He passes on this accumulated knowledge by writing articles about koi and koi ponds for magazines and for his website.

Syd now also teaches water chemistry for Koi Organisation International.

Koi Organisation International
The mission statement of Koi Organisation International is to promote and improve the International enjoyment of the Koi hobby by providing hobbyists with education and mentoring regarding good Koi husbandry, the recognition and management of Koi and pond systems problems and the management of improvement options.  Read more at:

Anoxic filtration has been developed over a period in excess of twenty five years by Dr. Kevin Novak PhD.  Whilst researching for an article about anoxic filtration in 2008 I found that Kevin had written a CD book about it so I emailed asking where I could buy a copy.  Kevin insisted on sending me a copy FOC and even refused to accept the refund of the Trans-Atlantic postage from the US to the UK.

I wish to express my deep appreciation to Kevin for the copy of his CD and for the help he has given to myself and to others via his forum posts and his blog website.  Thank you Kevin.

Kevin’s updated book is now available as a free download on iTunes:

Kevin also has FAQs and gives other useful information on his blog on this link: