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last updated 22nd October 2014

Ozone and its uses in Koi keeping

At Koi Water Garden, we are able to offer competitive prices on not only high quality Koi, but also on Koi related goods including ozone systems to provide the ultimate in water quality.


Benefits of Ozone in Koi Ponds


Ozone systems provide the ultimate weapon in the Koi keeper's armoury in the constant fight against disease and the struggle to maintain superb water quality. We have seen many advancements in Koi system filtration in recent years – new filters, new media, Bio UVs, new probiotic filter products all designed to improve water quality and filtration and thereby ultimately Koi health. None of these however comes close to delivering the benefits of Ozone systems, the advantages of which have long been recognised by marine hobbyists and like so many other great ideas, only now being accepted by the Koi world.  Without dispute, Ozone is the most effective natural bactericide and viricide of all disinfecting agents available to the Koi keeper.


 In our pond environment Ozone: -

  • Is highly effective in removing organics, and reducing ammonia and nitrites.
  • Is used as a sterilising agent to kill viruses, bacteria and other pathogens.
  • Is economical and non-polluting when used correctly.
  • Improves biological and mechanical filtration by burning off proteins, ammonia and nitrite straight to nitrates and by enriching our pond water with Oxygen.
  • Lowers biological oxygen demand (BOD) and raises the REDOX potential in the water (the ability of the water to oxidise pathogens).
  • Can remove toxic pollutants, such as hydrocarbons and other toxic substances from our pond water which cannot be achieved by any other form of mechanical or biological filtration.


 For our Koi ponds, in order to dose the pond water with the correct amount of Ozone we have to install an Ozone generator and a method of introducing the Ozone into the water in order to achieve the correct level of disinfection.


 This is achieved by passing air or Oxygen through an electronic ‘gizmo’ that generates Ozone by passing a high voltage electrical discharge through the air flow. The Ozone thus created is then drawn or pumped into a special protein skimmer, Ozone reactor or a 'reaction vessel' especially designed for the purpose. When the Ozone comes into contact with the water it quickly ‘burns’ off polluting organic material and breaks down back into Oxygen.


 As well as disinfecting the water and killing bacteria, viruses and free swimming parasites, Ozone also kills algal cells. The disinfected water returned to the pond is also saturated with oxygen so that the biological filter is able to work at its full potential. Ozone is so powerful that it is like having a dose of Chloramine T, Virkon or Potassium Permanganate in the pond, without any of the chemical side effects. As Ozone also burns off proteins and organics water clarity is improved enormously and the water sparkles to the point where the colour of the Koi is not impeded in any way by the depth of water, and Koi that have pure white skin at the surface also have pure white skin 6 feet down.


 In the example (below) the pond on the right does not use an Ozone system - note the air bubbles from the large air stone which look slightly brown or yellow in colour. The pond on the left uses an Ozone system. Note the blue - white stream of bubbles from the air stone. This helps demonstrate how Ozone removes the proteins from pond water which cause the water to look yellow/brown and murky. Text.

The pond (below) utilises an Ozone system to give the superb water clarity you can see, even from this low resolution photograph. The two Tanchos in this picture are swimming six feet below the surface.

Whilst not a Utopia, since even with the use of Ozone Koi can still need individual medication and treatment if required, the other major advantage of Ozone in our pond systems is that it virtually eliminates the risk of cross infection between Koi. So even if a fish is introduced into the pond environment which itself is ill, e.g. carrying a bacterial or viral disease, this is much less likely to be transmitted to other Koi. Further, any treatment employed to an infected Koi has a much improved chance of working effectively as the Koi are living in a much cleaner environment, biologically speaking – i.e. in an environment containing fewer harmful pathogens.  This is especially noticeable when topically treating surface wounds, which once cleaned, heal much faster and more cleanly. Similarly if parasites were present on certain Koi, the water would still need to be medicated to eradicate the parasites on the body of the Koi affected. Any medication used however, also works more effectively as the pond water is maintained at a much higher quality, with a higher Redox potential and with a lower biological Oxygen demand (BOD). Clearly the benefits of using an ozone system increase in a direct proportion to stocking levels, and in our case we have particularly noticed the difference when using ozone systems on highly stocked ponds of newly imported Tosai (one year old Koi). Tosai have much less well developed immune systems than adult Koi, and easily succumb to stress related issues caused by overstocking – leading to poor water quality, high bacteria count and the associated risk of parasite infestations. Since using ozone systems on our Tosai ponds we have noticed a substantial reduction in mortalities, infection and parasite problems generally. Last year over the course of the entire season, we lost only one Tosai from imported stocks of around 900. In previous years an average loss rate would have been in the order of 40 – 50 or 5%.


Frequently Asked Questions about ozone usage


What is Ozone?


Ozone gas occurs naturally in the atmosphere. When a molecule of Oxygen, O², is bound to a third oxygen atom, it becomes Ozone, or 0³. Ozone is an unstable bluish water-soluble gas with a characteristic smell. At low levels it makes the air smell fresh and its colour makes the sky blue. Ozone is the 2nd strongest known oxidiser and the most powerful readily available water sanitiser. It kills bacteria and viruses over 3,000 times faster and is a 50% stronger oxidiser than chlorine. It is unsurpassed for control of many types of common bacteria such as E. Coli and faecal coliforms as well as the de-activation of virus, fungus, mould, mildew and cysts, and is not carcinogenic. 


How does ozone work?


Ozone is nature's way of purifying the air we breathe. As ozone circulates and comes into contact with airborne (or waterborne) pathogens, one of the three oxygen atoms detaches itself from the Ozone molecule, attaches itself to the pollutant and oxidises it and turns it into a safer compound. Ozone is nature's way of cleaning our environment. Ozone is such a strong germicide that only a few micrograms per litre are required to demonstrate germicidal action; it destroys all pathogenic and saprophytic microbes in water. Factors like humidity, temperature, pH, Ozone concentration levels, type of organism and contact time, will affect the kill rate for pathogens, but the action of Ozone gas in water is instantaneous and after oxidation, Ozone returns to its original form of Oxygen, without leaving any toxic by-products or residues. Ozone is a natural disinfectant and steriliser and unlike chlorine, it does not produce trihalomethanes or chloroforms in water and so leaves no harmful toxins or residues in the water. Without dispute, scientifically speaking, Ozone is the most effective natural bactericide and viricide of all the disinfecting agents.


How do we measure the the effect of Ozone on the pond water?


The Ozone dosing of water is measured in millivolt terms and is expressed as the REDOX or ORP level (Reduction /Oxidation potential) of the water. Normal pond water will have a REDOX level of around 240mv or less. Sterile water has a REDOX level of around 700mv. Some Ozone generators are designed to automatically regulate the ozone dosing to keep the REDOX level at a preset level, normally around 350mv - 380mv in a healthy aquatic environment, so that the water is not sterile or anything like, since it could not then support life.


What is a Redox reaction?


A chemical reaction in which electrons are removed from one atom (which is thereby oxidised) and added to another (which is thereby reduced). The movement of electrons is then measured on a millivolt scale.  

What is Oxidisation?


Oxidisation is the process that causes steel and iron to rust, an apple to shrivel & go brown once cut open and is also responsible for the degeneration or ‘rusting’ of our bodies, causing cellular breakdown. Oxidisation or oxidative stress has been linked to many degenerative and chronic diseases including heart disease and cancer. It is also responsible for premature ageing. Oxidisation permanently disrupts and damages cell structure, thereby killing simple cellular organisms very quickly. Our beneficial nitrifying bacteria employ the process of oxidation to remove harmful ammonia and nitrite from our pond water by converting Ammonia to Nitrite and then Nitrite to Nitrate.


What causes Oxidisation?


Oxidisation is caused by oxygen molecules that are missing an electron, making them unstable. Oxygen cannot exist in this unstable form and has to be stabilised. The molecules collide with healthy, stable molecules be they in metal, or living cells and then ‘steal’ an electron in order to stabilise themselves. This damages and de-stabilises the molecule that they have collided with leaving it now missing an electron itself. The formally healthy molecule is now itself an unstable 'free radical' and will also try to stabilise itself by colliding with another healthy molecule and stealing one of its electrons. This creates a knock-on effect that damages and destroys living cellular structure, and in our pond environment this includes parasites, viruses, bacteria, algae or any other living organism with a simple cellular structure.  

If Ozone kills bacteria, what about my Bio-Filter - won't that be affected? Yes - but only beneficially! Pathogens are killed instantly as they pass through the ozone stream in the ozone reactor or protein skimmer, but obviously your nitrosammonas and nitrobacter do not, as they live on your filter media. True, they are free swimming as well, and any that pass through the Ozone system will be zapped. However, it is a recognised fact that the beneficial filter bacteria have relatively short lives but can multiply very quickly given ideal conditions and it has been scientifically proven  (Hueckstedt 1960) that nitrification bacterial populations increase faster in an ozonised system than in a normal non-ozonised system. This is almost certainly because the entire pond environment is cleaner and considerably more aerobic (oxygen rich) in an ozone treated pond, and it is a well known fact that nitrifying bacteria reproduce and perform much better in oxygen rich systems. Also some pathogens are anaerobic (live in a zero or reduced oxygen environment) and in a pond treated with ozone there will be virtually no chance of anaerobic conditions existing. So the high Redox levels of the pond water is of considerable benefit to our friendly bacteria, but is definitely the enemy of pathogens.


Extract from Professor Heuckstedt 1960 on the use of ozone in aquaculture. “It should be discussed whether ozone treatment and biological activities of bacteria co-operate. The suspicion that by ozone treatment not only the harmful but also the useful bacteria (nitrification bacteria for instance) could be killed is quite obvious. But strange enough this is unfounded. Either these bacteria are not in the free water but mostly at the bottom of the aquarium or on the algae or they grow faster than they die. Probably both versions are correct. It can definitely be said that nitrification bacteria will die faster without ozone than with ozone”  


If the Ozone kills only on direct contact in the Ozone reactor - what kills Algae and other organics that don't pass through the Ozone stream?


 Ozone raises the Redox potential of the water substantially. This renders bacteria, algal spores and other organics much more susceptible to oxidisation - to having their cells destroyed or disrupted, even though they are not necessarily in direct contact with Ozone. Put another way, chemically the pond water is far more reactive as it is saturated with Oxygen, and single cell organisms or organisms with a simple cellular structure are easily oxidised and therefore tend to have a shorter life span. Direct contact with Ozone in the Ozone reactor means instant death, the higher Redox of the pond water means cells are easily oxidised (killed) and cannot reproduce effectively so their population decreases.  


OK great, but if Ozone is so strong and effective at killing bacteria etc how do I know its not going to damage or kill my Koi?


Simple, the Koi never come into contact with ozone gas – only with water treated by ozone, remember, the water is treated, or disinfected by bubbling ozone gas through the pond water supply via a diffuser, or in a special skimmer or ozone reactor. Any residual ozone produced and not used up in the water flow gasses off to atmosphere and there is never more than a tiny trace of ozone actually present in the pond water. Ozone treatment leaves ‘free radical’ oxygen atoms available in the water which are responsible for the oxidisation process and which cause cell damage. Bacteria and protozoan parasites are single cellular organisms so they are relatively easy to kill – damage or disrupt one cell and you have effectively destroyed the organism. Koi are higher life forms and are made up of many billions of cells (like humans) and can afford to lose one or two, they do so every day naturally. Cells reproduce where necessary throughout their lives so there is no detrimental affect on our Koi as a result of using ozone any more than there would be by using the correct level of any established Koi medication in our ponds.


I've been told that Ozone makes the water sterile and can mean that the Koi's immune system deteriorates as it doesn't have to work so hard any more?


This is simply not true. In a sterile pond environment, the Redox level is around 700mv - at this level all life would cease to exist in the pond. The ideal Redox level of a Koi pond is around 380mv. At this level the pathogen levels are substantially reduced but your Koi will be very happy and healthy. Also there is absolutely no scientific evidence that disinfection of pond water by oxidisation, whether this be by ozonisation or chemical additives such as potassium permanganate, chloramine T or Virkon affects the immune systems of fish, including Koi. Remember, we are using ozone to lower the level of pathogens in the water, not eradicate them entirely, which would be virtually impossible – so the Koi’s immune system still has plenty of work to do when necessary. In their natural state, in rivers or lakes, each fish has a comparatively massive volume of water in which to live and bacterial loading is much lower than in a man made Koi pond. Using ozone simply helps us reproduce nature. It is like attaching your pond to a freshwater stream, with new water flowing in on one side, and old flowing out of the other - effectively continually flushing out the system. Would a Koi's immune system weaken in nature because it is not continually subjected to high bacteria counts? Of course not - quite the reverse! In our experience of using Ozone systems over the last 10 years we have seen absolutely no evidence that Koi removed from an ozone treated pond and moved into a 'conventionally' filtered pond are at any disadvantage, and we see no increase whatsoever in the incidence of disease because of moving Koi between these environments. Ozone is widely used in commercial fish farming applications in order to ensure the health and well being of fish fry and to enable the best possible water quality to ensure proper development of young fish. Marine and Tropical fish keepers have been using ozone systems to manage fish health for many many years and we do not see any detrimental affects to fish health – quite the reverse, so why should Koi be different? This is simply ill informed nonsense from the prophets of doom who simply can't - or won't accept ‘new’ technology, even when it works brilliantly.


Can I use an Ozone system in place of a conventional filter?


No, you should consider an Ozone system to be a part of your overall filtration strategy, as it will not replace either the mechanical or biological elements of your conventional filter, but it will enhance the performance of your existing system thus rendering it far more efficient.


Can I still treat my pond with chemicals? and what happens when I do?


Yes of course, from time to time you made need to use a pond medication for parasites etc. When you do, simply switch off your Ozone generator. Ozone would destroy chemicals in the pond very quickly and would render your treatment useless. When your treatment is complete, switch on again and the Ozone will clear the chemical residue very quickly.  


I have heard that Ozone is dangerous to human health - is this true?


Ozone is a powerful oxidising agent, and certainly if used carelessly or incorrectly it could be dangerous to health. However the same is true of almost all the chemicals we use in our hobby, even salt. Potassium Permanganate and Chloramine T are powerful oxidising agents and need to be handled and used correctly and definitely not ingested. Similarly Malachite Green, and Mercurochrome are dangerous substances which should not be handled with bare hands. Of course we know this and treat these substances with the respect they deserve. Indeed none of the substances we use in our day to day Koi hobby should be ingested, inhaled or come in to contact with bare skin - again common sense. Similarly we certainly don't want to breath in Ozone constantly as it can irritate the respiratory system, and therefore Ozone systems must be specified and installed correctly, with any residual Ozone produced gassing off to atmosphere - not inside your filter house! This is common sense, just as we should always install residual current circuit breaker devices in our electrical circuits anywhere near water - to prevent any chance of electricity and water mixing - definitely not a good idea! Yet we frequently hear from people who seem to have an inbuilt fear of 'new' technology, or simply because they don't understand even the basics of how Ozone systems work, that they must be 'dangerous' and should be avoided. Ozone has been in use as a safe and effective disinfectant for ventilation systems in offices, factories and even ocean going liners since the 2nd world war. It is used in modern bottling plants to disinfect plastic bottles before filling as conventional sterilization methods using steam would damage the containers.  It is now commonly used in wineries to disinfect bottles, casks and other equipment. Ozone systems are now also widely used in food production environments to keep food fresh longer and reduce food related odours. Ozone is also being used by some major water authorities to disinfect our drinking water instead of Chlorine simply because it is far more effective and is much, much safer. Ozone has been used in human medical treatments since the first world war. It is, and has been used for the treatment of MRSA and other bacterial issues that do not respond to conventional antibiotics. It is used for the treatment of Arthritis, Rheumatism, Hepatitis, skin lesions, ulceration and for a wide variety of internal infections and disorders. It has even been used as a cancer treatment. It is also now widely used in dentistry for disinfection, wound cleansing and improved healing. Download ozone use in medicine here to find out the scientifically documented facts - this will help anyone to dispel the myths. It is most unlikely that such systems would have been developed or used if there had been an unacceptable human health hazard. Ozone is used in many facets of industry, for many different industrial applications and no-one has ever died or been seriously injured as a result of ozone poisoning! The same cannot be said about most of the other commonly used chemicals that we regularly use in Koi keeping. World-wide, In over 100 years of ozone usage with some installations generating ozone at a rate of over 250kg/hour there is not a single record of inhalation of ozone causing even a single death - compare this with Hydrogen peroxide, chlorine and other chemical-based disinfectants, biocides and germicides... Even excesses of salt have been the cause of death...


Ozone has a very distinctive sharp, 'fresh' smell which many people identify with a disinfecting agent and it can be detected by the human nose at levels 10 to 30 times lower than that which is recognised as being harmful to human health. In addition, Ozone is very unstable, and once created, immediately begins to decompose back to Oxygen again. This decomposition takes between a matter of several seconds and several minutes. It does not therefore even exist in a 'dangerous' state for a significant time. We can therefore categorically state that a properly installed Ozone system would be a very safe part of your Koi filtration system and one with which there would be no associated health risks whatsoever. The prophets of doom will have to look elsewhere!


Can I dispense with my UV system?


In theory you can. However in practice we find it is best to leave the UV in place if you already have one fitted. Ozone generators have to be serviced from time to time and the UV can serve to keep the water clear whilst the Ozone system is switched off.   We would recommend leaving your UV in place if you are using dry air as an input gas, although for most of the year you wont need it! However once you start using ozone then you probably wont need a UV ever again!


Can I still use salt in my pond?


Yes, absolutely. The Ozone won't affect the salt and you will find that protein skimmers produce a lot more foam with salt in the water, so it actually works even more efficiently.


All sounds very complex - do I really need an Ozone system?


No, we can't claim an Ozone system is an essential part of a Koi pond filter system, but that's what was said about bottom drains and mechanical pre-filters such as sieves or vortexes a few years ago. Now very few 'proper' Koi ponds are built without these essential items. For the serious Koi keeper we would recommend an Ozone system be included as part of the overall filtration strategy. We believe that in the near future, Ozone systems will become just as much of a necessity as a bottom drain. Koi are beautiful but expensive creatures and each year thousands of Koi die needlessly from all kinds of illnesses most of which are simply caused by poor water quality - nothing more. In any Koi pond one of the eternal and recurring problems that we have to overcome is bacterial disease and in a well stocked and mature Koi pond the bacterial load on the system can become very high - especially in the summer months. Without doubt the single biggest benefit of installing an Ozone system is that the bacterial load on your system (and therefore on your Koi) will be drastically reduced. Ergo less disease - more healthy Koi. Unquestionably water quality will also be transformed using Ozone. Ask yourself why have marine/tropical aquarists been using Ozone to help manage water quality for the last 20 years or so? Why is the Koi fraternity always the last to catch on? The saying that we are not Koi keepers - we are water keepers is absolutely true. If your pond water quality is superb, your Koi are more likely to be healthy and live longer - it's as simple as that.


What comprises a complete Ozone system?


 We need several items to build a complete Ozone system, rather like building your own Hi-Fi system. First we must have an Ozone Generator to create the Ozone initially and optionally a Redox controller which enables the system to operate automatically and switch on and off the Ozone supply as required. In most larger systems we then need some kind of Protein Skimmer, ozone reactor or mixing chamber to mix the Ozone produced with the water as efficiently as possible, allow it do 'disinfect' the water, and then allow any residual Ozone to decay before the water is returned to the pond. We can also install an oxygen generator, as Ozone systems utilising oxygen as a feed gas are super efficient. Without doubt however, the most important elements of any Ozone system is the efficiency of the Ozone mixing system employed coupled to correct installation techniques. But Like any good Hi-Fi system, it is pointless purchasing a very high quality amplifier if the speakers you will be using are poor quality. Similarly you can spend a fortune on a high quality CD player but if the amplifier is wrongly specified or poor quality you won't get the performance out of the system that you should. You get the idea, it's the same with Ozone systems - you need to match the Generator to the mixing system and other system elements to achieve the desired result. At the smaller end of the scale, the Ozone reactors and skimmers tend to be very efficient, and this therefore means that the amount of Ozone we need can be reduced because the reactors are very good at mixing water/Ozone for optimum performance. Efficiency can be increased further by the addition of an oxygen generator. At the other end of the scale, we need to produce more Ozone than we strictly need with air driven systems as the ozone reactors are not as efficient when scaled up, when larger volumes of air are used, and more ozone gases to waste as mixing is less efficient. To overcome this issue, mixing systems employed on larger systems tend to be of a different type or design in order to ensure efficient mixing - for example venturi injection and static mixers. Oxygen generators can be retro fitted to just about all ozone systems to massively increase efficiency./p>


The technology sound complex, are these systems reliable?


Early Ozone generation equipment was not particularly reliable, as the generators produced a lot of heat, and early systems were water cooled. These were large and cumbersome and needed fairly regular maintenance. The latest systems are all air cooled, are far more efficient , use less power and therefore produce less heat energy. The smaller systems , like our own use relatively tiny amount of electrical energy and are simple and very reliable devices which are guaranteed for one year. There are no moving parts in the generators other than cooling fans and the only maintenance required is an annual clean or replacement of the generator electrodes. Even the larger, more sophisticated units in our range are air cooled but still use a comparatively small amount of electricity and there is therefore little wasted energy to convert to heat anyway. Again these units need minimum maintenance. The only moving parts are the cooling fans. These are also guaranteed for one year. All of our Ozone systems are easily maintained. Cells in the ozone generators normally have an expected life of around 4000 hours of continuous use , or approximately 1 year of normal usage where the unit would be expected to be switched on and off by a controller or timer - before replacement is required. Costs of servicing an ozone generator vary between £95 and around £160, or around the same as replacing the tube from a Bio UV annually. Oxygen generators would need new molecular sieves every 4000 hours of continuous use, again around 1 year, assuming they are switched on and off as required in normal use. The protein skimmers and ozone reactors have no moving parts (other than adjustable valves) and need no maintenance - not even cleaning. The only item which needs more regular attention relates to systems utilising Redox controllers where the actual Redox sensor should be cleaned with a soft toothbrush about once per week to prevent it fouling.


Ok, I'm convinced, but how do I choose the right system?


Firstly, you obviously need to know your pond gallonage - then consider some of the factors which will influence the ozone requirement by studying the tables below. From this you can calculate the size of Ozone generator you will need by using the formulae of between 0.4gm/hour and just over 1gm/hour of Ozone for every 1000 gallons if using dry air as an input gas. When using oxygen as a feed gas, via an oxygen generator you will only need 0.1gm - 0.2gm of Ozone per 1000 gallons.  


Start Point Using Air as a Feed Gas Comments
Recommended Dosing 0.4gm Ozone / 1000 gallons (4500 Litres)  
Add / thousand gallons 0 gm 0.1gm 0.2gm Add the amounts (left) to the starting dose for each parameter
Stocking Density Low Medium High Average stocking density is 1 fish inch for every 10 gallons (45 litres)
Pond condition Clean Average Dirty The More detritus and sludge in the pond, the higher the BOD (biological oxygen demand.
Planted No Somewhat Heavily Plants, including Algae use oxygen and therefore raise the BOD
Temperature Less than 10 deg C; 10-15 deg C Above 15 deg C The higher the temperature, the faster the metabolism of all organic life forms so the higher the BOD
Start Point Using Oxygen as a Feed Gas Comments
Recommended Dosing 0.1gm Ozone / 1000 gallons (4500 Litres)  
Add/Thousand gallons 0 gm 0.05gm 0.1gm Add the amounts (left) to the starting dose for each parameter
Stocking Density Low Medium High Average stocking density is 1 fish inch for every 10 gallons (45 litres)
Pond Condition Clean Average Dirty The More detritus and sludge in the pond, the higher the BOD (biological oxygen demand.
Planted No Somewhat Heavily Plants, including Algae use oxygen and therefore raise the BOD
Temperature Less Than 10 deg C 10-15 deg C Above 15 deg C The higher the temperature, the faster the metabolism of all organic life forms so the higher the BOD


We can obviously help by recommending a suitable system for your given application - please e-mail for further information at  


Ozone Systems

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