Why become a koi keeper?

Over the last few months I have asked a wide range of koi keepers that same question and although most of their answers were different, I was surprised by how long it took each of them to reply with their definitive answer. The fact that I received so many different reasons shows that people keep koi for a range of motives and that once you have started keeping koi, it has many associated attractions. Summarising, people keep koi because they have so much to offer.

A sample of the reasons why people keep koi. Where do you stand?

Assembling a collection is a life’s work.

There are always new koi to add to my collection

My koi collection is a way of expressing myself. * They are relaxing to watch

Koi keeping opens up so many other interesting areas. * Status symbol

The pond cam first, then the koi * Koi keeping allows me to use my practical skills in the garden (Pond construction, design etc.)

Unlike other hobbies, a koi collection can never be complete

The challenges and responsibilities of maintaining my koi in tip-top health.

The vast range of sizes, colours and characters offers so much promise. * There is always something I can be doing that is koi-related, if it’s only reading up on things.

On exploring these answers a little deeper, the appeal of koi can be attributed to specific aspects of the koi itself. The hobby centres on the differences between koi and other pond fish available and the challenges and rewards associated with koi. On the one hand, the ‘kings of the pond’ are just like other fish but they are also different from other fish in many ways. It is these differences that make koi so popular and also dictate how we should keep them.

To a casual observer, perhaps viewing a range of pondfish, including koi, in an aquatic centre, besides price and variety available there is little difference between the two. They are available in similar sizes, displayed in similar vats and on appearance fed similar food. So why change these practices when koi are brought home to a garden pond?

True, koi have key requirements similar to other fish, a balanced diet, suitable water quality and a sensible stocking rate but they have different environmental requirements as dictated by their own biology and if these are not provided for then our achievements in koi keeping are likely to be limited.

Probably the most common way people get the koi bug is by seeing them in other people’s ponds or by keeping a few mixed in with other fish in their pond. Soon they realise that the small garden pond is not sufficient to achieve the full potential of that handful of beautiful koi and they begin to think of adapting their existing pond to a koi pond. Very rarely can a garden pond be adapted due to its small size. A totally new installation is required, usually along the lines of a more regularly shaped, deeper pond.

This is a very contentious step to take as it can quite reasonably be argued that a ‘typical’ koi pond is not the best environment in which to keep koi. The natural mud ponds where algae and muck abound produce and maintain koi in a far better condition than those that we can achieve in gin-clear koi ponds. Koi ponds are designed and maintained as a compromise between the requirements of the koi and those of the koi keeper. A compromise between the two cannot by definition be best for both the koi and the koi keeper.

Many koi keepers successfully rear their koi in ‘mixed’ ponds where they share the pond with other pond fish such as goldfish, orfe, shubunkins, ghost carp etc. I have seen vibrant koi in such ponds, that are only 2 feet deep and are filtered with a submersible pump-fed box biofilter. These tend to be the exception rather than the rule.

The recommended conditions for keeping koi in optimal health is a stable, healthy water quality and this is best provided with as large and deep a pond as possible.

Just because these are recommended features of a koi pond does not mean to suggest that it is only possible to keep koi in a pond of such dimensions and specifications. The main benefits to a koi keeper of having such a large pond are that they make it easier to provide a stable water quality and a large surface area to improve growth and reduce competitive stress.

Larger water bodies heat up and cool down relatively slowly compared with small ponds and they allow a wider margin for error or ‘comfort zone’ when feeding and stocking with koi. Compare the ease of keeping a goldfish in a pond compare with the problems of keeping it in a goldfish bowl.

Before embarking on a ‘conversion’ there is plenty of advice available on how such ponds can be built and the options available with the budgets to match. Spend lots of time in research and planning and seek lots of advice. However, bear in mind that one of the most common remarks for a koi keeper to make a few months having built a pond is ‘I wish I’d made it bigger!’

Why don’t other fish require a similar pond design?

Actually, most pond fish would benefit from the stable conditions provided in a koi pond but they do not require it as their genetic make up makes them more resistant to extremes and variances in water quality and temperature. Compared with koi, goldfish are not too far removed and inbred from their more vigorous and ‘wild’ carp-like ancestors and are able to withstand environmental changes that could easily stress and cause disease in other weaker varieties.

The problems associated with inbreeding and vigour can be seen in the extreme through the inbreeding of goldfish where ‘fancy goldfish’ such as orandas, ranchus, bubble-eyes etc have been produced. Just like koi, these fish are selected for their individual visual impact rather than their vigour. However, compared with koi these are inbred to such an extent that they would not likely to survive a UK winter in a garden pond.

What is the ideal koi pond?

When posed with such challenging questions that are central to the hobby, and where the extensive debates will mean that koi keepers people are likely to disagree on the answer. It can be quite useful to go back to the fundamentals of the hobby and the one thing that is common to all koi keepers, the koi itself. There is even real practical value to be gained from thinking like a koi. It is only really then that we can identify firstly, what our koi and then secondly what we as koi keepers require from a koi pond.

A koi’s view of an ideal koi pond.

If koi were to design their perfect pond then they would look no further than a carp’s natural environment. They have become perfectly adapted and at home in such conditions over many generations. There would not be a pond-liner in sight, the water would more likely be a murky mixture of green and brown and there would be a soft and sludgy clay substrate. Koi heaven!

The koi’s wild ancestors prefer still, open waters where they are free to ‘graze’ off the pond bottom, rooting through the silty substrate for a steady supply of food. This is also true for koi and this is how they are most successfully cultured and grown in Japan and across the world. They thrive in such stress-free conditions where the clay pond’s ecosystem is its filter and the soft clay substrate is an abundant source of both interest and food. The clay gives the water chemistry an excellent stability and the murky water reduces the stressful threats from above where they are able to feed from the bottom in a naturally low stocking density.

This is not the koi keeper’s typical pond as we know it. However, it is quite common for ‘show koi’ to be put into natural clay ponds for a number of months for them to benefit from natures pampering conditions.

A koi’s view of a typical koi keeper’s pond.

The koi keeper must be able to see the koi. Clear water is provided through the installation of a UV clarifier and a pump and filter system to keep the water quality ‘sweet’. Often limited by space and budget the pond is likely to be a few thousand gallons, but with a stocking density way in excess of what is found naturally. Consequently, pump and filter turnover times must be increased and what is a still-water fish is now experiencing strong currents from the pumped returns and any up-welling currents from diffusers.

There is no soft substrate in which to root around but a smooth ‘hygienic’ liner or fibre-glassed bottom and the absence of clay minerals can cause problems with aspects of water quality and koi health. This can be addressed by occasionally adding clay to ‘refresh’ the system which turns the water milky temporarily until the filter removes it again. Hardly natural!

At feeding time, the sedate lifestyle of a bottom feeding fish is interrupted by a feeding frenzy at the surface where if any of the koi do not feed and compete then they are likely to miss out. To some extent, the koi are forced to feed like predators rather than grazers.

I recognise that in order to make the point, the differences between naturally and artificially kept koi have been addressed in quite an extreme way and most koi adapt well to artificial koi ponds. However, there is little debate as to which of the two environments is more stressful and why at times, medication is an integral part of koi keeping.

If you were a koi, which would you choose?

Recognising the above criteria, unfortunately one of the consequences of keeping koi are health problems. The main cause of koi health problems and ultimately koi deaths is our mismanagement of the pond environment. Each problem will have a cause that can be traced back to a human error, accident or mistake.

The main cause of problems with koi and which can ultimately lead to heartache and significant financial loss is stress.

This is an all-embracing term for the fish’s response to a change in its environment which it is unable to tolerate. The fish’s biological response to stress leaves it susceptible to disease and other undesirable side-effects.

Stressors in the pond could include:

Water quality:

Excessive ammonia, nitrite or nitrate.

Poorly carried out water change


Low dissolved oxygen


Heron/Predator attention

Excessive activity above the water surface


Insufficient food

Deficient food (lacking vitamins etc.)


Competitive stress

We cannot eradicate disease, but we can prevent it from increasing from a natural background phenomenon to a lethal epidemic by managing and maintaining a stable environment.

A useful way of visualising the issues of stress and koi health compared with that of other pond fish such as goldfish, orfe and ghost carp is with the seesaws below.

In Figure 1 only a small amount of stress is required to affect a ‘move’ in koi health whereas in Figure 2 with other pond fish that are less in-bred and more vigorous, they are hardier in that more or prolonged stress is required before fish health is affected.

In conclusion, one of the reasons given by a koi keeper when asked why they keep koi was ‘the challenges and responsibilities of keeping koi in tip-top health’. For me, this sums up the appeal of koi keeping and fishkeeping in general when we are rewarded by a pond full of healthy fish, returning our investment of time and energy. This is more so with koi keeping where the odds are stacked against us from the start. With many aspects of life though, the greater rewards follow the greater risk and this should be true for more koi keepers, especially those who take time to think like a koi!

Koi are artificially produced ‘carp’ in an artificial environment. Hardly a natural combination and at time a marriage far from heaven. In such conditions what will you have to do to keep koi healthy?

1. Spend lots of time watching your koi.

This may sound too easy and obvious but it is time well spent. On a farm you will often see a shepherd or a stockman leaning on a fence just watching their stock. To many people it would look as though he is having a rest or being quite lazy but ‘stock watching’ is vital work in any husbandry as changes in animal behaviour are often the early signs of more significant problems. The earlier such changes are spotted the better the chances of limiting any associated disease problems.

We can often fall into the habit of only watching our fish when we feed them. However, fish do not exhibit their natural, everyday behaviour during feeding and can mask subtle changes in their behaviour. Be sure to watch your koi when they are unaware of your presence to be able to observe some typical koi behaviour.

2. Test water regularly.

The water parameters that are most likely to threaten your koi are ammonia and nitrite. These can quickly get out of hand, particularly in a new system or after additional fish have been introduced. An ammonia or nitrite reading indicates that the fish are producing waste quicker than it is being broken down by the filter and the water is becoming toxic and dangerous to the fish.

In such cases, stop feeding and carry out a partial water change.

In more mature systems, the build up of nitrates can be a problem so these must be tested for and when excessive, diluted with a water change.

The pH must always be between 7 and 9 and a pH of between 7.5-8.5 is ideal.

3. Adequate nutrition

Koi prefer to feed a little and often. Feed them a recommended and reputable quality pellet. Also recognise that you are not just feeding your koi but your filter as well. Even though your koi may not appear to be producing a lot of waste, the highly toxic and colourless ammonia is excreted through the gills.

4. Ensure that a pond is designed with stability in mind.

Koi prefer stable conditions. A pond should be designed as deep and as large as is practicable as this will help to stabilise water temperature and water quality. The solution to pollution is dilution.

5. Filtration

The most common weakness in a pond filter is its mechanical filtration or its ability to remove solids. Ensure that areas allowed for mechanical filtration are sufficiently large enough and easily maintained and cleaned. Solids removal is also greatly influenced by how and where the water leaves the pond (pump-fed, bottom drain, surface skimmer etc) and the size and path of the pipework.


If koi prefer a natural mud pond to a typical koi pond then why do I need the following equipment in a koi pond?

1. Pump.

This is essential, as the pond water needs to be circulated through a filter. If the water were not circulated through a filter then there would soon be a build up of toxic wastes produced by the fish. Aim for the pump to circulate the whole pond volume once every 1-2 hours.

2. Filter.

A filter performs a number of vital roles.

a) To remove solid particulate matter such as fish waste and other debris from the pond. b) To act as a biological filter where a media with a large surface area maintains a colony of beneficial bacteria that break down toxic fish waste.

3. UV Clarifier

A UV clarifier is required in a koi pond to kill the algae causing green water to make the water clear. This is only for our benefit as koi much prefer and indeed thrive and ‘colour-up’ better in green water conditions. 4. Aeration

Koi in an artificial pond are likely to be kept at a higher stocking rate than in the wild and require extra aeration to supplement that which diffuses naturally from the atmosphere. Extra aeration also benefits the filter bacteria which require considerable amounts, especially in the warmer months. An indication that extra aeration is required is when koi are lethargic or hanging near the surface.

Jargon Buster

Ecosystem. The natural interactions and food webs found in a pond that keep the environment in balance.

Kill blanketweed and string algae.