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last updated 23rd February 2022

How to Keep Koi successfully



Having spent all that money on your favourite Koi, ensure you keep them happy and healthy - on this page we tell you the secrets of how do just that.


Koi health - the secrets.


You have built your pond, read all the Koi books and on-line material, and then chosen and purchased some wonderful Koi - but how do you ensure their long term health? First - The golden rule of Koi keeping - As Koi keepers we are water keepers first and fish keepers second.


If you can achieve and maintain near perfect water quality, you stand a very good chance of keeping healthy Koi with a minimum of problems. 80% or more of problems that are encountered with Koi husbandry are associated with poor water quality, and we therefore make no apologies for dedicating the majority of this article to the subject of achieving and maintaining prime water quality.


Koi are pedigree animals, just like pedigree horses, dogs and cats. They are highly inbred in order to achieve the quite magnificent range of varieties colours and patterns that we see today. Because they are highly inbred they are genetically weaker than their plain ancestors - the same is true for any pedigree animal. As they are genetically weaker, they are therefore more prone to disease and have weaker immune systems than common carp.


Understand these simple and basic facts from the start and you have already begun to appreciate the basics of Koi keeping. Koi are related to Gold fish, but will not tolerate the same water conditions as Goldfish or many native species of cold water fish. Gold fish will survive in tiny ponds without filters, in stinking water with little or no attention, fresh water, food, oxygen or human attention. Koi will not - Koi need prime water quality and lots of care and attention for their well being - they need a substantial investment of your time for their very survival.


What do we mean by prime water quality?


We mean water with zero or negligible levels of Ammonia, zero or negligible levels of Nitrite, a stable pH of between 6 and 9, a minimum oxygen level of 6 mg / litre, an absence of all common toxins that we can regularly find in tap water, for example Chlorine, Chloramine, Iron, Lead, Zinc, Copper, and many other pollutants and above all water with stable chemistry. In other words, water parameters which are stable, do not change rapidly and can be maintained in a stable state - this includes temperature.


Koi also need relatively hard water with a KH and GH of at least 6 degrees of German hardness or 100 mg/litre dissolved salts (mainly, but not exclusively calcium and magnesium salts) Harder water is beneficial in several ways to Koi, its helps stop any toxins dissolving in the water, and 'locks' them up in a form that can not so easily affect our Koi. It also aids osmoregulation, the phenomena by which fish balance the fluid levels in the body. See our article on KH and GH for more detailed information.


Please do not make the common mistake of assuming that your water quality is great because the pond is clear, or bad because it is murky. Pond water does not have to be crystal clear to be of high quality - battery acid is clear but it wont do your Koi much good! Green water full of algae is of fantastic benefit to Koi as it brings out certain colours very well and contains many nutrients.


How do we achieve the desired water quality?


Achieving ideal water quality is not necessarily that easy. We start with a raw product provided by our municipal water companies that is far from perfect - it is arguably not all that great for humans, but without some attention it is certainly not suitable for keeping Koi. So firstly we MUST incorporate a professional water purifier based on active carbon technology to treat the incoming water for pond filling and any large water changes to ensure the new or raw water added to our ponds is as good as we can make it.


The pond itself must be of an adequate size to house our proposed Koi collection. How big does a pond need to be? - Very difficult to quantify, but I would submit as a good rule of thumb that you should aim for a minimum volume of 1500 gallons (6800 litres) with a depth of 4ft minimum. Ponds with a smaller volume than this are more difficult to keep Koi in, not just because of their physical limitation of size and space but because it can be very difficult to achieve stable water parameters in small ponds. Everything can change too quickly, especially temperature and oxygen levels. Small ponds can be very successful, but they will need more attention than larger ponds to achieve the same result.


Clearly it is also much easier to overstock a smaller pond , and even with the best intentions, Koi can grow fast, and a pond that was set up with a well balanced population can quickly become overstocked within a couple of seasons. Overstocked ponds create unstable water chemistry in that the most dangerous pollutants, Ammonia and Nitrite can rise to dangerous levels very quickly, thus creating immediate and real problems for the unsuspecting Koi keeper. So the number one rule with pond sizing is always build it as big as space and budget will allow - this you will never regret.


Having decided on the size of pond, you will need to ensure you provide an adequately sized filter. We cover filter sizing in some detail on our 'how to choose the right filter' page so we will not repeat ourselves in detail here, but suffice to say that the right size of filter for your pond will be probably be between 10% and 20% of the surface area of the pond - at least in conventional filter terms. Black box filters from your local Garden centre or Aquatics chain are a definite no no if you are serious about keeping Koi. A good Koi filter will be capable of removing the majority of solids from the water and of biologically processing the pond water adequately in order to remove all traces of Ammonia and Nitrites under all conditions, and it is not uncommon to find that a Koi pond filter system will represent at least one third of the cost of building the pond in the first place.


Whatever else you do don't cut costs when choosing the right filter for your pond. Koi ponds don't become ideal homes for Koi overnight, even if they are correctly sized, specified and built with the most fantastic filter. A Koi pond take at least 9 months to mature properly, and your Koi will need even closer monitoring (or rather your water quality will) during this period. This is because as a pond matures, it will establish a healthy aerobic bacterial population within the filter (but also on the walls and floor of the pond) that will convert the toxic Ammonia produced by your Koi to Nitrites. As the Nitrite levels build up, a second bacterial population will breed which oxidise the Nitrites to harmless Nitrate. This process takes time - as a general rule it will take around 8 weeks at 16 deg C to build up a healthy population of Nitrosammonas bacteria, which convert Ammonia, but it will then take at least a further 6 months for Nitrobacter to breed in sufficient numbers to neutralise the Nitrites.


You can speed up this process in several ways, firstly by using a proprietary filter start product which contains a culture of Nitrosammonas to kick start the filter. You can also seed your new filter with filter media from an existing mature pond, if available, which will also serve to speed up the maturation process. You can also add several 'suicide squad' fish as we call them - which may be of lesser quality or more hardy and can withstand poorer water quality, in order to speed up the process before you introduce your expensive purchases. Regardless of all this you MUST then test your pond water regularly, at least twice per week, for the presence of toxic Ammonia and Nitrite, whilst the filter is maturing. Even with mature systems, you should never become complacent about water quality and should carry out weekly checks on these two vital parameters.


Koi never stop growing, and whilst their growth will be slower in smaller and more heavily stock ponds, they will never the less put on weight. As they grow they will consume more food, and excrete more Ammonia. Over time the efficiency of any filter system can be compromised through a build up of detritus and mulm within it, and the pond itself can also become host to a considerable amount of decaying matter, be it dead bacteria, algae, weed, leaves and other detritus or even decaying uneaten food. This can increase the amount of Ammonia produced, and can compromise an otherwise efficient filter system. Regular monitoring of water parameters will therefore provide valuable information on the health of the environment as well as directly on the quality of the water.


A proper regime of pond and filter maintenance is also therefore of paramount importance in keeping water quality in tip top condition. Depending on the size and type of filter installed, maintenance might be a daily, weekly or monthly requirement. More maintenance will be required in summer months than in winter, although perversely it is much safer to carry out major filter maintenance in winter than in summer, as we should disturb the filter biomass (the friendly bacteria living in our filter media) as little as possible.


Depending on its design, the pond might need hoovering regularly to keep the pond base free of detritus and bottom drain pipe work, Vortexes and settlement tanks will all need regular purging in order to prevent the build up of mulm.


Why the obsession with water quality?


We have already explained that Koi are pedigree fish, just like horses, cats and dogs are pedigree animals. Owing to generations of inbreeding to obtain the desired physical traits - colour, pattern, size, body shape and even variety, Koi are genetically weaker than their ancestors, the common carp which means their immune systems are weaker and they are therefore more susceptible to disease. Poor water quality causes stress in Koi, and the increased stress levels further suppress an already weakened immune system. This in turn means that both parasitic and bacterial disease are more prevalent in stressed Koi, and so poor water quality can be directly and indirectly responsible for fish ill health. Stressed Koi produce less mucus, and it is the healthy mucus coat on coldwater fish that acts like a disinfectant and can prevent disease. The mucus coat hinders the movement and reproduction of parasites and forms an antiseptic barrier against harmful pathogens including a range of bacteria and fungal problems. Once the mucus coat is breached or becomes damaged or ineffective, then the Koi will surely succumb to one or more problems very quickly. Excellent water quality means zero stress - zero stress means that the immune system is working at full tilt - this means maximum protection from disease.


To demonstrate just how important good water quality is, if you are treating a sick Koi for any type of disease, it is virtually impossible to cure the problem and return the fish to full health in poor water conditions, even if all the correct treatments are being used. An ideal example of this is the often misuse of quarantine tanks to treat sick fish because there is an obsession to isolate sick Koi. However, all too often quarantine systems are not adequately matured, are too small or hopelessly overstocked - result poor water quality - often resulting in dead Koi. For the most part it is better to treat sick Koi in their main home, where water quality is often much better than a hastily set up hospital or quarantine facility, and the risk of disease transmission between Koi is often overstated.


The correct diet


To keep your koi healthy and happy, the correct diet is essential. There are now a large number of specialist Koi foods on the market and all are designed for one or more of the following - health, growth, colour and boosting the immune system . Having spent considerable sums on our Koi, please don't compromise their health by feeding them a diet of trout pellets, goldfish food, bread or other titbits! Koi need vitamins, proteins, carbohydrates and trace elements in their diets just as humans do for health and well being and all modern Koi foods are specially formulated to provide the right balance of these elements.


As Koi are Poikilothermic (their body temperature is governed by their surrounding water temperature), they also need a change in diet through the seasons, low protein foods in winter and at low temperatures and high protein foods in summer at higher temperatures. At low temperatures, food passes through the simple gut very slowly and is poorly digested. Food that stays in the gut too long can start to decay and make the fish very ill, and also poorly digested food means more waste, which means more Ammonia which means poorer water quality.


In winter you should only feed Koi once per day maximum and normally at temperatures lower than around 8 deg C (45 deg F) depending on the size of your pond, you should stop feeding altogether. However in mid summer, at high pond water temperatures you can feed your Koi up to 8 times per day. The golden rule is that any food you give should be consumed in under a minute. If there is still food floating around after this time you are almost certainly feeding too much. If in doubt reduce feeding - remember more food means more Ammonia and Nitrite - and as far as we know no-one has ever killed a Koi by underfeeding, but it is very simple to kill a Koi by overfeeding.


As well as the excellent range of pelleted foods on offer, you can and should offer treat food, especially in warmer water conditions. Food such as lettuce, oranges, sweet corn and prawns will all be devoured with gusto, once Koi learn to recognise the new food. Initially you may find Koi refuse new types of food - but they will acquire new taste quite readily especially if they are used to a mixed diet. You should never just use the same pelleted diet all the time as Koi can get so used to just one diet they may then refuse all other types of food, including just different formulae pelleted food. So vary the diet and keep them interested!


See our how to feed Koi article for more information.




The other crucial factor in keeping Koi healthy and happy is observation. You will learn to understand the patterns of behaviour of individual fish, as well as the shoal. Koi are individuals, who will behave differently and react to stimuli in different ways. Some Koi will always be shy feeders, swimming around the bottom in deep water, some will always be first up for food, readily feed from the hand, and swim round at the surface all day long.


Like humans Koi don't all behave the same way, and you will learn to recognise these individual traits. If the behaviour of an individual Koi changes, especially if the change is sudden, then this change should be viewed with some suspicion. There will be a reason for the change which may be perfectly innocuous, but may be early signs of the onset of a problem. If the behaviour pattern changes, think what you as the Koi keeper have changed - have you undertaken some large water changes, changed the diet, introduced new Koi etc. If you have made no significant changes to your regime of husbandry, then possibly, just possibly, the individual Koi may be stressed or even ill.


Keep a close eye on the individual, especially at feeding time. If the Koi is still feeding then generally there won't be much seriously wrong. Even so, when the Koi comes up for food make as close as visual inspection as possible to see if there is any obvious signs of damage, change of colouration or stress. Carry out a full set of water tests to ensure that your water parameters are as good as they should be. If the Koi starts to swim by itself, sit at the bottom, even in warm water conditions, or refuses food then it is generally time to remove the Koi from the pond for a closer inspection. With small Koi this can often be achieved in an ordinary inspection bowl. With larger fish, you may have to anaesthetise the Koi to ensure you can handle it with minimum stress (to you as well as the Koi).


Examine the underside of the Koi for damage and any lesions, with the Koi upside down in the bowl, you can quite easily inspect the gills for damage and discolouration. A healthy Koi will have burgundy or plum coloured gill membranes, which will be in continuous 'layers' with no obvious physical damage. If the gill membranes are pale, grey or pink and have frayed edges then this could be sign of gill disease. However, this can also often be simply ascertained by a process of observation whilst the Koi is swimming. If for example it is obviously panting, or has rapid or erratic gill movements, or constantly swims or floats near to a waterfall or water return then this is a good sign of oxygen deficiency which can be caused by gill disease (as well as a number of other factors).


Whilst the Koi is in the bowl it is also a good idea to take a mucus scrape which can then be examined under a microscope to establish if the Koi is carrying any parasites, and if so what the parasite is. The presence of a high number of parasites on a Koi can lead to gill and skin damage, a thickening or even discolouration of the mucus coat, and will always eventually cause the Koi to flick or rub itself on the pond walls or floor or other projections. Parasites can also make the Koi become lethargic and can eventually kill if left untreated for too long. See our article on how to identify and treat parasites if they are determined by a scrape.


If the Koi has obvious damage on the body, this could have been caused by some physical injury such as colliding with a projection in the pond, or even from the unwanted attentions of a Heron or other predator. If the damage shows obvious reddening and / or raised scales then this is probably an early sign of bacterial disease in the Koi itself which can be treated with a topical antiseptic such as Iodine whilst the Koi is still bowled. Serious bacterial problems may need specialist medications or even antibiotics in order to effect a proper cure.


If the scrape shows no signs of parasites and the water tests undertaken demonstrate that the water quality is good, and there are no visible signs of damage on the Koi's body then there could be an internal problem such as incipient organ failure - however these are virtually impossible to diagnose fully as we can't practically take our Koi to the local Vet for an X-ray or Scan! Happily, most problems can be determined by following the simple steps outlined above and this will also then give us a clue as to any remedy that we might need to apply, either to the individual Koi or the pond itself.


However, or more importance, we need to try to understand what has caused the problem in the first place - what has made the Koi ill. In over two thirds of cases I would say that poor water quality is the root cause of the problem. Improve the water quality and the Koi stands a good chance of curing itself.


Sometimes, individual koi can become diseased or attract parasites for no apparent reason, even if the water quality is excellent, and then we need to treat the pond with an appropriate medication and perhaps the Koi itself. However, indiscriminate use of medications is to be avoided at all costs as this will definitely spoil the prime water quality you have worked so hard to achieve and unless you apply the right medication for the condition/parasite in question you will only be making matters worse.


For a comprehensive list of Koi medications and how to use them see our article on the subject. Of course treating Koi for different ailments is a very wide subject in its own right and it would be almost impossible to cover the subject fully, and provide the detail required in these pages, however, if the golden rules above are adhered to, then health problems WILL be minimised.


The last piece of good advice we can offer is as always basic common sense - but before you buy your first Koi ensure your pond environment is up to the job. Then buy a good water purifier, a Koi inspection bowl, a suitable net and a sock net to catch any Koi which needs your inspection. Follow these simple steps and watch your Koi grow and thrive and maximise your enjoyment of this wonderful hobby.





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