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last updated 2nd January 2022


Myths and Misunderstandings in Koi Keeping

More on the what not to do as well as what's best for your Koi. Don't be mis-led by out of date and incorrect information.



Koi Keeping - Fact and Fiction


There is so much information about Koi keeping now available in Koi books, DVDs, internet blogs and on the internet generally that it can be really difficult at times to get to the 'truth'. For sure in many aspects of Koi keeping there are no single right ways of doing things - but there are always many wrong ways that can and do lead the unsuspecting novice Koi keeper into difficulties. Unfortunately some of the information and advice you will hear/see/read will be either taken directly or indirectly regurgitated from old publications, and in addition many of the 'opinions' voiced by the inexperienced comes from nothing more than folk lore or from 'friends' who knows someone who knows someone ...... In addition and very unfortunately at least one of the two main Koi magazines currently published continues to print articles by so called 'experts' on the subject who simple don't know what they are talking about. This is particularly frustrating for those of us who have been involved with the hobby for many years as very often the novice or newcomer to the hobby will start by subscribing to such publications in the hope of being properly educated on the subject. Technology, including the internet, have led to a much better knowledge of Koi keeping techniques generally and a much wider availability of good equipment and advice worldwide now means that many of the original knowledge, techniques and methods used 20 - 30 years ago are out of date and simply no longer true. So here we try to up date the reader on the most common and widely publicised myths and misconceptions in the hope that the explanations given below will help educate the unwitting.


Soft water is better for keeping Koi


We disagree. Koi should be kept in pond water with stable chemistry, and especially a stable pH level which can be between 6 and 9 and a stable KH and GH level above DH 6 (Degrees of Hardness) or 107 mg/litre. KH is a measure of temporary, or carbonate hardness, or Alkalinity, and GH is the measure of general hardness and represents the total of all dissolved minerals in your pond water. Test kits generally measure the amount of calcium and magnesium salts present representing the GH which is very difficult to change , but KH can easily be changed unwittingly as well as intentionally.


There is a view among some that a low KH and GH  (very soft water) is good for Koi development which encourages growth and help to improve colour development. This is in fact at odds with the science which confirms that Freshwater fish, especially the carp family, should be kept in a stable environment with a KH and GH no lower than DH 6 – as above. Water much softer than this renders the process of osmoregulation, the ability of the fish to pass much needed salts into the body and expel water and Ammonia back into the pond, much more difficult, and it is a scientifically proven fact that ‘harder’ water, with a KH and GH above DH 6 is much better for the health of Koi. This is because as osmoregulation becomes more difficult (in softer water), this leads to stress in cold water fish, including Koi, and raised stress levels lead in turn to a weakened immune system and potentially therefore to the onset of disease.


Soft water – with a low KH will allow dangerous swings in pH which is then easily changed by events such as an acid rain storm, or even a large water changes. The pH of water controls the pH of the Koi's blood, and Koi are VERY sensitive to changes in their blood pH


 Hard water also locks up toxins more readily and helps to stop them affecting Koi directly – this is especially true with metals, and in soft water metals such as copper , lead and zinc will dissolve very easily. KH (carbonate hardness) is also an important source of energy for nitrifying bacteria that eliminate ammonia and nitrite.


Harder water therefore also helps the biomass in the filter system to reproduce and flourish more easily, which helps reduce free Ammonia and Nitrites in the pond water. In addition there is absolutely no scientific evidence that Koi grow any faster in soft water, nor is there any similar evidence that soft water improves Koi colouration. However it is an accepted theory that the red colouration of Koi is maintained better in softer water with less tendency for red colouration to breakdown, weaken or to develop black – ‘shimmies’ as they are known.


Unintentional changes in KH and GH can occur in pond systems using ceramic filter media such as Bakki house or Blagden Pro ceramic media, for example, as all ceramic medias appear to strip calcium from the water , which can dramatically lower both KH and even GH levels. If you use ceramic media in your pond then we would advise monitoring the KH/GH levels and buffering with tailor made mineral packs as required.



You should always use Hydrogen Peroxide immediately after a Potassium Permanganate treatment.


Why? This idea appears to have come from one particular manufacturer/distributor of Potassim treatments and was put forward with the objective of halting the progress of a PP treatment at any chosen point and secondly with the purpose of adding more oxygen to the water.


The question is why would you want to stop the progress of the PP treatment in the fist place? Presumably you added this treatment to kill off a parasite infestation or to lower bacteria count - so why stop it?. If the treatment is to work then it must be allowed to run its full course. 


Yes its a good idea to add more oxygen during a PP teatment as this chemical does strip a lot of oxygen from the water - but why use another chemical to do this when adding oxygen via more airstones or another air pump is a lot simpler and safer.


The fact is that Hydrogen Peroxide is yet another powerful oxidising agent, and having just used one powerful oxidising agent (PP) - you then choose another eqully dangerous medication whilst the first is still active to stop its reaction?? - so if you are halting the PP treatment because you are afraid of overdosing your Koi or damaging them becuase of the PP treatment - you potentially are only adding to your woes by using another powerful drug!! Hydrogen Peroxide is just as likely to damage or even kill Koi as PP.


The correct approach is to be sure that if you must use PP in the first place that you use the correct dose and then allow it to do its job. Add extra oxygen by all means to the water via another airpump/air stones - but dont for Godsake be tempted to throw in another chemical at any stage to kill off the first - you will be playing with fire....



You should turn off your filter system in Winter


What utter and very dangerous nonsense! If you turn off the filter in Winter, the living bio-mass it contains will die, as you have switched off its food source (Ammonia and Nitrite) and starved it of oxygen. When you switch on again in Spring, you will be right back to square one with a dead filter, and you will run the real risk of your Koi developing all kinds of problems over Winter and in Spring as your pond water conditions will be very bad indeed. You MUST always keep your filter running 24 x 7 x 365 days of the year – if you cant afford the electricity too run your filter all year round then DON’T KEEP KOI.



You should turn off your filter system when treating your pond for parasites or bacteria


Another no no – if you have an uninvited parasite population in the pond, or an unhealthy population of pathogenic bacteria which you are forced to treat by adding chemicals – these nasties will also be present in the filter. To rid your system of all nasties – you MUST treat the entire system, including your filter. The unwitting are afraid that the chemicals used will damaged the filter bio-mass – so they turn it off during treatment. It is true that a good disinfectant such as Potassium Permanganate or Virkon – also Formalin - are generic chemicals which are indiscriminate and can kill all bacteria – good as well as bad – and therefore in an undersized, dirty or immature filter the bio-mass will be affected. However the effects are only temporary and a good filter system will quickly recover from any chemical treatment, especially if the Koi are not fed for a few days during the treatment regime. If you don’t believe that parasites are present in the filter when found on Koi in the pond, then take a ‘scrape’ from a water snail – always found inside filters – and I guarantee you will find it covered in the same parasites that are affecting the fish



You should turn off your air pump in Winter to stop the air cooling the water


Nope – wrong again – air pumps run very hot – try putting your hand on the base of one for a few minutes and see if you can toast your fingers! Some of this warmth is passed into the air supply and into the water so the pressurised air won’t be any colder than the ambient air, and will almost certainly be a lot warmer. In addition the movement of air and water in the pond will help prevent ice forming on very cold days and nights..



A Venturi is the best and simplest way of adding oxygen to your pond


Afraid not.- venturis went out of fashion years ago, but some still insist they are a must in a Koi pond. Nowadays we use air pumps, low wattage devices designed to run 24/7 which are specifically designed to force air (with its oxygen) into the water, generally at low level, thus giving the oxygen carried in the air supply ample time to diffuse into the water. Venturis create a substantial backpressure on the water pump used, reducing its flow by as much as 60% and shortening the pumps life. Unfortunately many dont realise this and buy a water pump in the expectation that they will achieve a given flow rate and therefore a proper turnover rate. In fact flow rates can be so drastically reduced that the filter system just wont do its job properly. In addition venturis only work just under the surface, they wont work deep down, and therefore any air they do produce doesn't have time to get properly dissolved into the water anyway..



You should leave a new pond to mature before adding fish


Er- No ! The pond wont mature at all unless you add some fish! Fish will produce Ammonia, which will in turn encourage the growth of a population of Nitrosammonas bacteria in the filter – these will multiply and gradually oxidise the Ammonia to Nitrite – this will then encourage the growth of a further population of Nitrobacter bacteria, which then flourish and oxidise the Nitrites into harmless Nitrates. This maturation process will normally take up to 9 months to complete – a typical filter system will mature for Ammonia within 6 weeks, but can take up to 9 months for Nitrite..



You should dip your new Koi in a suitable chemical bath before introducing it into your pond


Oh God please don’t! If you buy your Koi from a reputable Koi only dealer – and you should NEVER buy Koi from a Garden centre or Aquatic chain – then the Koi will have been properly quarantined and treated for parasites and rested for at least a month to ensure that it is healthy when collected and free of disease. Dips are very dangerous to use – especially for the inexperienced Koi keeper. Chemicals doses, water quantity used and exact timings are vital to ensure that you don’t damage the Koi beyond repair – a few seconds too long in a dip that’s a little too strong will almost certainly damage, stress and possibly do irreparable damage to the Koi. More Koi are unwittingly killed each year by the improper use of dips than just about any other way..



Tap water is safe to use in Koi ponds


No it is not – not without some form of purification anyway. Yes you can get away with using tap water for months or even years – and many people do – but sooner or later you will run into problems. Each year we get a number of clients who report to us that they have lost large numbers of adult Koi after performing large water changes using untreated tap water. This is because they are not aware of what toxins are potentially in our water supply. These include, but are not limited to :- Chlorine, Ammonia, Chloramine, Aluminium Sulphate, Sodium hydroxide, and Orthophosphates to name but a few. Most of the substances and chemicals added are toxic to fish, including Koi, in quite low concentrations. Chlorine is especially toxic to fish (and to humans too by the way) as it forms hypochlorous acid in the water and damages fish gill tissue, mucus and delicate finnage very easily. The amount of Chlorine added to drinking water also varies enormously, not only from area to area, but can also change on almost a daily basis. For example , in mid Summer, and over a bank holiday weekend , water authorities will tend to ramp up the amount of Chlorine added to water supplies as water demand increases, and the temperature rises. If an authority is carrying out major repairs to water mains and pipe work infrastructure locally to you, they will almost certainly dose up the water supplies with extra strong doses of Chlorine and other disinfectants to ensure that any pollution created by the repair work is neutralised. Carrying out large water changes on your pond during times such as these can and often does lead directly to Koi mortalities. Such repairs also commonly release large amounts of Iron (in the form of rust) into the water supply which is, like most metals, very toxic to Koi. This is just one example and reason therefore to ensure that you ALWAYS purify water intended for the pond using a good quality activated carbon based purifier. The cost of such a purifier is around the same as one good quality two year old Koi. It’s a no brainer.  



Herons can't get the Koi in my pond... it's much too deep


Yes they can! Herons are extremely resourceful creatures and we have not found a pond yet that is Heron proof (other than those completely enclosed indoors). Herons can fish Koi from deep ponds just as easily as they can in shallow ones. If they cant reach them from the sides, they will jump in and swim just like Ducks – and take fish whilst swimming. Unfortunately Koi are so greedy for food that they will come up to the surface if there is any movement by the side of the pond or at the surface to see what all the fuss is about and to check if there is food about to be given. Herons are very patient and if not disturbed will sit for hours awaiting their chance. Netting a pond is also no guarantee of safety as Herons will jump onto the net to push it under the surface of the water if possible. They will also enter buildings with ponds inside to get at the fish. They will also learn to avoid electric fences and other deterrents. What is more they also tend to arrive at dawn and dusk when Humans are not around so are often not even seen by the pond owner. Please DON’T use a plastic Heron and stand this by the pond – as it will only ATTRACT a real Heron ! We only know of one effective deterrent so far – and it normally comes with two barrels – but unfortunately its not legal! .



Koi will sit at the bottom of a pond in Winter because it is warmer there


No, that’s not the reason – In Winter when water temperatures are low, a Kois metabolism will slow right down in response to the colder water and reduced light. Koi are Poikilothermic – that is their body temperature is governed by the temperature of the water in which they live. They will rest on the bottom to conserve energy, and feed much less during Winter, and sometimes they will fall right over on their sides in response to very cold water temperatures. However, this is nothing to do with seeking out the warmest part of the pond. In a natural environment, a lake with virtually no water movement the phenomena of thermocline exists – where warmer water sinks to the bottom of a body of water in Winter and fish respond by spending most of their time at these levels. In a Koi pond, thermocline does not exist – even in very deep ones, simply because our ponds are artificial environments where we are turning over the water in our pond through the filters every few hours , so constantly mixing the water – so the water at the bottom of the pond is exactly the same temperature as the water at the top. Koi might be hoping for warmer water in the depths, so spend time down there hoping to get warm – but they are always going to be disappointed.


I know my pond water is good because it's as clear as crystal


Yes – well so is battery acid but its not much good for Koi. Water quality has little to do with clarity. The pond can be pea green and murky but the water quality may be excellent. Good quality water means water containing zero levels of Ammonia, Nitrite, Chlorine, Chloramines, metals and all other pollutants commonly found in tap water and with an oxygen content of at least 6 mg / litre, a stable pH of between 6 and 9, a kH and GH of 6 or above and a stable temperature. The measurement of water quality is in fact very difficult as there are so many trace elements and substances that can be found in water other than pure H2 0 – but one things for sure you cant judge water quality as good  just because you can see the bottom of the pond..



Pumped filter systems can be just as good as gravity systems


Very, very rarely is this true. The logic of why gravity systems are so much better than pumped systems is very simple. In a gravity system the foul water from the pond travels through a large bore (normally 4”) pipe very slowly into the filter so the detritus carried into the first stage of the filter remains intact and in relatively large chunks. Fish poo, uneaten food, weed , algae etc. This means that the first stage of the filter will remove or trap much of these solids very easily and they are easily flushed to waste. The mechanically cleaned water passes through to the biological stages of the filter -where the friendly filter bacteria live – and these more easily colonise the filter and flourish in clean water rich in oxygen – so the biological stages of the filter will work as designed. Then the pump processes clean water through the UV, which will also work at 100% as the water is clean and the UV light can penetrate as designed.


 In a pumped filter system the pump in the pond picks up the dirty water, full of detritus, and macerates or liquidises this as it gets pumped up through the UV and into the filter through a small bore tube where the water moves at a relatively high velocity. So both the UV and filter receive oxtail soup rather than cleaned water. This means the UV wont work as well, as the light cant penetrate the dirty water so well and this will need to be more powerful to achieve the same result. The filter cant cope with the oxtail soup and tiny suspended solids well even if it is well oversized. Small micron sized suspended solids are not easy to trap and do not settle out so dirty water gets through into the biological stages of the filter. The media in the biological stages gets and stays dirty so the filter bacteria don’t colonise or flourish as well, and the oxygen supply will be limited as the media gets dirty. So as well as the mechanical stage not working well the biological stages are now much less efficient too and to cap it all much of the suspended solids not trapped in the filter are now passed straight back into the pond meaning that water clarity is also spoiled. This means that pumped filters also need much more maintenance, need more powerful UVs so are more expensive to buy and to maintain. And even then the water quality and clarity will never be as good with the pumped system as with the gravity system. It’s a no brainer.



Aerated bottom drains are the best way to aerate a pond and improve the efficiency of the bottom drain


A debatable issue – but in our view the big problem with aerated drains are:- · More expensive to buy and to plumb · Need at least annual maintenance to keep the diaphragms clean – which means a swim in the pond · Owing to the large airflow through them, the lids tend to float off very easily – more swimming · The airflow from the drain does not, as is often claimed, help pull dirt into the bottom drain, but rather encourages dirt to be raised into the airflow from the drain and into the water current so making the pond murky. If you want a healthy air supply in the pond, use a large circular ceramic plate air stone which don’t clog easily, can be moved easily to find the best position (normally in a corner of the pond – NOT over the bottom drain), and can be pulled out altogether when required for cleaning. It’s a simple system that’s cheap, practical and it works. .



It's best to turn over the volume of the pond through the filter at least every hour


Great if your pond is 500 gallons, but not such a good idea when its 5000 or even 50,000 gallons unless you happen to own your own electricity generating company! Yes one can argue that fast turnover rates are needed in small ponds – when its actually difficult to turn over the water slowly anyway, but biological water quality is always more difficult to maintain in a smaller pond as its much easier to overstock and water chemistry can quickly become destabilised.


The fact is however in any Koi pond the no 1 objective is to process, or turnover the volume of the pond through the filter at a sufficient rate to remove all traces of Ammonia and Nitrite from the pond water. As the pond gets larger the turnover rate can actually get slower and on average a 2 – 4 hour turnover rate is quite sufficient to achieve super water quality. This is easy to test with Ammonia and Nitrite test kits. If the filter is mature , of a sufficient size and the turnover rate is sufficient there wont be any Ammonia or Nitrite measurable. If you do get significant Ammonia and/or Nitrite readings in the pond then speeding up the turnover rate wont help if the filter is of insufficient size, and a higher water flow will actually be detrimental as the filter then wont get as long as it should to process the water flowing through it. 


So if you are seeing constant Ammonia/Nitrite readings in the pond, this will be more to do with overstocking , or under filtering – and simply increasing the turnover rate through the filter almost certainly wont help. In addition, and especially in larger ponds, a healthy proportion of the biological stage of your filter will be the surfaces of the pond, the pond walls and floor, which will be colonised by the same filter bacteria as found inside your filter. The bigger the pond, the greater the surface area, so the bigger your bio-filter becomes. This is another reason why turnover rates become less important, as pond water is in contact with all surfaces of the pond at all times, the filter is working at the same efficiency regardless of turnover rate.


Again this effect increases with pond size. So essentially speeding up the turnover rate will increase your energy consumption, may look better aesthetically if you like watching Niagra falls in the back garden, but will almost certainly do little to improve the water quality.


As an example of this thinking, we have a 33,000 gallon growing on pond, quite heavily stocked for much of the year. This is filtered by one large trickle tower processing a water flow of around 2000 gallons per hour – so our turnover rate is something like every 16 hours ! Yet the water quality is perfect, 0 Ammonia and 0 Nitrite despite the pond being fed with an auto feeder 4 times per day on a high protein food.


Growth rates in this pond are exceptionally good with baby Koi growing from 4” (10cm) to 8” (20cm) in 6 weeks, and on average to 12” (30cm) in 16 weeks. Please tell me why we need to turn over the water any faster?.



You can't use Potassium Permanganate or Formalin with Salt


Well yes, actually you can. Any medications that we commonly, or typically use today (2022) are safe to use with a salt concentration of up to 0.33% (1/2 oz per gallon, even Formalin and PP - provided that your water hardness (KH) is 6DKH or above.



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