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Anoxic heading

The latest anoxic filtration book in an easier to read format is still free and is available on iTunes

For the latest news and developments in anoxic filtration systems visit Dr Kevin Novakís blog


Anoxic filtration notes

Anoxic filtration is an entirely different concept to conventional biological filtration since it removes ammonia from the pond without adding nitrate as an end pollutant.  Although anoxic filter systems are very simple to build, they must be built as described or they are unlikely to produce good results.  Iím frequently asked to explain how anoxic filtration works, but itís impossible to explain it in a just few paragraphs so anyone intending to build the system should read up on how to do it beforehand rather than risk being disappointed with the results of incorrectly chosen materials.

As a guide, there should be about one basket per adult fish for the system to work correctly, just putting one or two baskets of cat litter somewhere in the pond may not have any significant effect.

Kevin Novakís book explains anoxic filtration in detail but it can be a hard read for those who are unfamiliar with scientific expressions.

To make the system easier to understand for the average koi keeper I wrote a much simpler description in a two part article for Koi Carp magazine. Kevin has included that article in the second issue of his CD book under the heading of "Read Me First". The ďRead Me FirstĒ section is well worth the read because it not only contains my article as a simple introduction to the main part of the book but it goes on to give the details of other koi keepersí setups.

You can read my simplified article as it was published in Koi Carp (in two parts) on my website:

In the US, where Kevin lives, suitable clay based cat litters are common but in the UK there are many different types and manufacturers keep changing brands and ingredients so you will have to look around to see whatís available in your area. Below are types that have retained their structure in tests and have been used successfully but you should check that the composition hasnít changed before buying large quantities.  Obviously donít try to use any 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.

Tesco premium (wet)03

Tesco Premium (wet)

This is the granular type of clay that you should be looking for.  The colour isnít important and it need not be little round granules but it mustnít 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.

You need a clay based cat litter that is granular and which keeps that structure when it is submerged.  You should rinse out any dust but very fine litter that packs down tightly like wet sand or litter containing too much dust to rinse out wonít be suitable. Itís preferable not to use scented litter but if you canít find a suitable unscented one then a scented litter can be used if the perfume is thoroughly rinsed away.  Products described as ďClumping Cat LitterĒ are invariably useless because, when they get wet, they just go to a soggy mush.

Some people have found that a product called Oildri or plain Fuller's Earth are consistently suitable types of media. 

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 looks suitable, 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 to make all your baskets, complete with Laterite, and use them straight away in full confidence.

Types of cat litter that have been reported as being successful

Steetley Sophisticat Original (Cropped)03
Steetley Sophisticat Original

Tesco low dust cropped03
Tesco low dust cat litter

Tesco odour control04
Tesco odour control cat litter

Tesco Premium03
Tesco Premium cat litter


Laterite 103Laterite 204Laterite
Search around for the best price - some users claim to have found it on the Internet for as little as £16.00 for 1.5 kg (2012 prices).

Use about 200 gm to 400 gm per basket depending on the basket size and, as a guide, for effective stand-alone anoxic filtration you will need about one basket per adult fish.


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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 not develop a root system that will grow so large that they will burst the basket you can choose plants you like the look of and they will be fine.

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