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Dissolved Organic Carbon (D.O.C.) in Water and its Effect on Successful Koi / Fish Keeping.

In general terms proficient aquarists and pond-keepers are well used to analysing water quality using PondLabTest Kits for such parameters as pH, nitrite, nitrate and ammonia and in many cases GH / KH (Hardness), Salinity and Oxygen. These assessments, most revolving around the nitrogen cycle, are a direct result of metabolism of food by fishes and represent an indication of loading of the pond and capacity of filtration to cope with that input from the livestock supported (biological oxygen demand or B.O.D.) and represent useful indicators or markers as to the proficiency of the system to cope with the loading of fish and livestock.

Feeding the fishes results in metabolism of the diet by the fish and creation of waste solids plus ammonia compounds which are then converted into itrites by bacteria (Nitrosomonas) and thence further to nitrates (Nitrobacter). However there is always more to the issue than when you try to talk simply about a complex question…particularly in science!

One such complication is the build up of dissolved organic carbon or D.O.C. Such things as phenolic compounds, polyphynolics, lipids, tannins (many of which add to taints and coloration as tell tale signs).

There have been studies made on the actual involvement of mud and detritus in the complete aquatic environment as being an active site of decomposition by bacteria of organic and inorganic dissolved carbon compounds – the detritus itself being involved in adsorbing the compounds before the bacteria can then convert them.

Similarly, pond keepers are more than conscious of the requirement for surfaces where bacteria and slime can accumulate for full bioactivity, such as on the surface of filter material and structures involved in the pond.

So what is D.O.C. and where is the source in a pond?

Dissolved Organic Carbon is a term that covers a multitude of organic (carbon compounds created by life) materials which end up dissolved in water produced by incomplete digestion of foods and decomposition of plant life. Some are able to be broken down (bio-degrade) whilst many are termed refractory and are neither bio-degradable nor removable by conventional treatment methods. As a consequence they are sinister in their accumulation particularly as their increasing presence puts demands on available oxygen in a system – in preference to and in excess of the demand from your prize possessions, the Koi.

What are the signs of high D.O.C?

In many cases we are unaware of their presence which would only be witnessed by a well-trained eye noticing a discolouration, surface foam, a lack of vitality or purity of the water or, worse and often irreversible, a change in behaviour of the fish. More generally we see:

Growth of blanket weed or thread weed, duck weed in uncirculated / aerated ponds or stagnant water, green water micro algae utilising the nutrients and taint/odour to the water.

Ulcers and unexplained illness/lethargy not associated with identifiable parasites.

Unexplained mortality. / Loss of appetite and vitality.

Unfortunately we cannot easily test the actual D.O.C. but we can look at indicators of its presence by measuring the reduction potential of water (redox) or Oxidation/Reduction Potential (ORP).

Because the DOC’s are often organic acids their accumulation can result in reduction of pH and unstable conditions.

How can we remove D.O.C.?

1) Dilute out, (physical)

2) Adsorb and dispose (physical)

3) Neutralise (chemical)

4) Physical removal – foam fractionation (physical)


1) Dilution:

Regular partial water changes will of course assist the removal of D.O.C. by dilution but this is not a complete answer as there will always be some D.O.C. left in the remaining water. Pulling the bottom drain and allowing detritus to go to waste reduces the production of dissolved organics in the circulating water as too does cleaning a settlement chamber or primary matting / brushes regularly but this is more like good housekeeping rather than a remedy.

2) Adsorbtion:

It is possible to adsorb (actively soak up the organic material into a physical adsorbent material such as granular activated carbon (G.A.C.) which has a defined capability to adsorb after which it dumps back all that it has withdrawn, so use of G.A.C. can be dangerous if not changed regularly. I would personally

recommend at east 2Kg / 1000 gallons and changing every 4 weeks minimum and only use top grade pelleted carbon.

A superior alternative is fine grade montmorrilonite clay (NT labs: Koi Care Mediclay)

Mediclay acts as an electro-chemical -adsorbent drawing in the charged compounds onto the molecules of aluminosilicate compounds that make up the clay this material when used properly dissipates throughout the water in a cloud and then is drawn to the filter or bottom drain taking with it the chemical waste attached by the charge. This process is called tyndalisation and results in a return of a sparkle and depth of clarity.

It has the beneficial side effect of adding trace elements that are of benefit to koi which are generally used to rooting around on muddy pond bottoms and so the clay is a dietary benefit (hence why NT use Koi Care Mediclay as a unique ingredient in Medikoi Beauty Food)

Use of Koi Care Zeolite Chunks which is removable to re-charge is beneficial although in heavily stocked systems the quantity may need to be more than recommended for controlling ammonia peaks.

Similarly we also market a product - Pond Booster which is based on fine zeolite and helps by trapping the organics and can be incorporated into a filter system like Zeolite Chunks

3) Neutralisation:

This can be done by use of chemicals which actively destroy the organic content, bonding and structure of the compounds, one such method would be the use of Ozone gas, which will oxidise the compounds and cause their accelerated decomposition and destruction, it does have the serious disadvantage of being a dangerous material to use in a system, being extremely harsh to humans especially through mucous membranes (eyes, nose etc) and any detectable ozone smell should warn the user to switch off the ozoniser and remove him or herself from the presence of the gas, used in water it is less easy to detect although one can link control of production of ozone at an ozone generator to a controller system of redox measurement (Redox meter) thereby producing what is necessary to break down the DOC linked to its detection , but you must be certain of the accuracy of the measurement and the design of a contact tube (such as the NT Tornado Protein Skimmer) modified to prevent the Ozone entering the aquatic system , preferably using granular carbon to filter any ozone before the water returns to the pond.

Another chemical used to neutralise organics is Koi Care Permanganate Dip (potassium permanganate) at the 1 ppm dosage treatment recommended for "pond cleaning " adding 10 mls / 400 litres (100 gallons) results in rapid increase in ORP (oxidation/reduction potential) and destruction of the DOC, the change in colour from pink to brown denotes the use of the characteristic of the potassium permanganate to oxidise organic material (something it also does to bacteria and parasites on the fishes when used in therapy- when a stronger dip in clean tap water (low DOC) at 10 ppm ( mg/litre) level is necessary for 30 minutes exposure).

4) Physical removal:

Using a foam fractionator or protein skimmer such as the NT Tornado Protein Skimmer / fractionator – sometimes referred to as bioprotein skimmer.

This equipment creates foam – something that we indicated could be observed in pond as a side effect of having a high DOC – and uses this characteristic to help remove the organic compounds.

Why does the foam arise and how do the compounds leave with the bubbles?

The foam is created using a high sparge of air bubbles injected into the water through the venturi and then trickling the water over the high surface area Biocubes. DOC compounds are composed of molecules with hydrophilic (attracted to water) bodies and hydrophobic (dislike water) chain ends this results in one end forming in the bubble membrane whilst the hydrophobic section is drawn into the bubble,this high affinity of the compounds to be drawn to the air bubble means we can actively pull them out of water without removing the water or having to attach them to an adsorbent material and so in long term use the skimmer / fractionator uses nothing other than circulation water, energy and air and has no other on-costs.

Benefits of low D.O.C.

No algae, no blanket weed, no colour or taint in water – see the true colour of your fish.

No growth restriction (some of the hormones prevent fish developing) reduced nitrates, stable pH.

Reduced infection risk, reduced stress on fish, increased longevity and vitality.


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