Koi pond construction

When considering the design and location of a garden pond, it helps to take into account the type of pond you want to build.  This will determine the design and final location for the new pond.

If our aim is to encourage wildlife by providing an aquatic oasis, then the necessary features would be reflected in the design.  Similarly, if our ambition is a pond of colourful fish, then we would adopt a different design to the wildlife pond, incorporating a filter, a pump and more open swimming space.  Further along the spectrum of ponds for fish comes the koi pond, where the health and welfare of the fish are at the centre of our considerations, and this is firmly reflected in the ponds design.

The objectives of designing a koi pond

A koi pond is designed and built with displaying the koi in a clear and healthy environment being the main objective.  Everything is planned with the koi in mind. 

As koi are unlike any other pond fish, the design of a koi pond is quite unique. 

1.  Koi grow to be large fish

Koi can grow to over 1 metre in length and require their accommodation to offer wide open swimming space.  By providing as large a pond as possible (both in area and depth), then koi growth and development will not be impeded, allowing your koi to reach their full potential.

2.  Koi are notoriously weak. 

Koi are the product of many generations of selective line breeding, where closely related fish are crossed to exhibit the desirable features, such as pattern and colour.  An unavoidable phenomenon associated with such a high degree of in-breeding is the reduction in the vigour of koi, particularly the more delicately patterned and high grade specimens.  Essentially, koi are bred for their external appearance, and not their internal vigour. 

Keeping koi is not as nature had intended, where the fittest survive.  So when we keep koi we play a key role in making sure the weakest survive, and to do that we need all the help we can get – which starts with a well designed koi pond. 

Because koi are weaker than other pond fish, they must be protected from stress, – the precursor to health and disease problems.  As water quality is the most common cause of stress, the koi pond must be designed to provide your koi with the best water conditions.  The larger a pond, the more stable an environment your koi will experience.  Larger ponds hold more volume which will be less prone to rapid temperature changes and will also play a large diluting effect on the build up of pollutants.  Koi will thrive in a pond where conditions are favourable and stable, they cannot tolerate rapid change and have a habit of letting you know if they do. 

3.  Maintaining ideal water conditions.

Koi keepers have a reputation of wanting to show off and compare their filter systems just as much as their koi, and rightly so, it is an essential part of the pond’s design.  A filter system and a koi pond are inseparable and when designing the ideal environment for koi they should be considered together.  A filter will maintain the clarity of pond water while removing and breaking down the toxic fish waste.  It is the engine room of the pond, and the better it is designed the better it will perform, giving your koi superb water quality and your peace of mind. 

4.  An unplanted pond

The first impressions when viewing a koi pond are its clean lines, where the sides are vertical and shape is simple and uncluttered.  A koi pond can be quite clinical in appearance, looking unnatural in its unplanted state.  Plants and koi do not make good bedfellows and the decision to go plant-free should be made early on.  Aquatic plants are potted or basketted up in soil which is topped off with a layer of gravel to retain the soil and weight the plant to the bottom.  To koi, the aquatic pigs of the pond, a row of planted baskets presents the same temptation to pigs smelling out truffles.  They’ve just got to root around – it’s their instinct, and they won’t stop until they have removed, tasted, and stirred up the aquatic soil, uprooting plants until they float to the surface.  Not really a recipe for a tranquil, crystal clear pond.  So plants are not an option.  This will mean that green water may soon take hold (unless controlled with a UV), but if the pond is crystal clear, then blanket weed will thrive with its monopoly on sunlight and nutrients.  This can be controlled with the strongest of algicides with no need for any consideration for other aquatic plant life.

5.  Go for depth.

Besides being steep sided (there is no need for planted marginal shelves), koi ponds are the deepest of all garden ponds.  Building a koi pond is back breaking work because of the extra spoil that is to be removed (and this is why many leave it to the professionals).  An absolute minimum depth of 3 feet should be provided for koi, with many being 6 feet deep and over.  A little ingenuity at the design stage can save some digging by raising the pond above the ground,  (2 foot above the ground and 4 foot of digging!).

Considerations when going so deep will include:

            1)  A 6 foot deep pond will need an 8 foot deep hole to allow for foundations and pipework (bottom drain for filter)

            2)  Drains.  When purging the filters that sit alongside the pond, ensure that there is sufficient fall from the bottom of the filter to the main sewer.  This will allow the easy draining and cleaning of the filter chambers throughout the life of the pond.

What Equipment will you need?

1.         The Pond.

Due to the size and depth of a koi pond, using a preformed pond (fibreglass) is ruled out, leaving either a liner or a concrete construction.  Of these, the easier DIY option is a liner (PVC or Butyl), with construction expertise required every step of the way for a concrete or blockwork pond.  This is particularly true if fibreglassing is used.  

Liners can be cut to any size, once you have excavated your pond.  They can even be box welded to fit the hole snugly, removing the chore of having to gather and hide the folds of liner.

Concrete blocks can also be used to construct a very robust pond.  This is the only reliable method of constructing a deep pond as it prevents the vertical sides from caving-in.  The concrete blocks can then either be lined with a liner or covered with a smooth render that can be waterproofed with several coats of black G4 liquid plastic, or fibreglass (a potential nightmare which is best left to the experts).

One of the advantages of building a pond from blockwork is that the filter can also be constructed simultaneously, adjacent to the main pond.  Using blocks, rather than purchasing a prefabricated filter gives you a very free hand in designing the filter, just how you want it. 

If constructing with blockwork is not an option then there is an ever increasing number of preformed filter systems that simply need lowering into position and plumbing into the pond, ideally through the bottom drain(s) on the pond bottom.

As koi have a tendency to be eager feeders and grow at a pace, a significant area of the filter should be set aside to removing the solid matter. 

The first chambers of a filter will be dedicated to mechanical filtration, taking out the solids by settling them out or by entrapment using brushes or other easy-to-clean media.  The essential feature of a mechanical chamber (and other chambers as well) is that they can be emptied with ease, flushing any settled matter away to the main sewer.

The biological activity of the filter is responsible for breaking down the waste that is toxic (but invisible).  The beneficial bacteria that colonise the filter media will mature into a large population that will keep the water quality sweet.  If purchasing a ready made, multi-chamber system, then media will be supplied.  If you construct your own filter, then a wide range of media are available to choose from.  These include:  filter matting, Alfa grog, flocor and other perforated plastic media that are easy to clean.

2.  Pump.

The pump will circulate the large volume of pond water, servicing the filter and keeping the bacteria alive.  Choose the pump wisely, one that offers the performance you require (turning your pond volume over every 2 hours minimum), and carrying a lengthy guarantee.  As this will be at the heart of your koi’s life support machine, and running continuously, it will be prudent to check its running costs.  Look out for a low wattage pump. 

3.  UV.

Similar to any other UV used in a garden pond, it will guarantee crystal clear water.  For larger koi ponds, multiple UV units will have to be linked to perform their task efficiently. 

Because koi tend to be so much more valuable than other pond fish and they require a little more knowledge to appreciate how to optimise their health and well being, it would be worth investing in a good book on koi.  Besides the points already covered, a good book would cover vital topics such as koi nutrition, water quality and health care.  Even though the design and construction of a koi pond is carried out with the koi in mind, great consideration should be shown to the koi themselves, so that you can complement the healthy environment your koi are experiencing adequate care and husbandry.  Designing an effective pond and filtration system, although essential to successful koi keeping is only the first step on the road to successful koi keeping.

Koi  (Nishikigoi) – Cyprinus carpio.


Koi or nishikigoi (Japanese for coloured carp) originate from the paddy fields of Asia and Japan. Like most ornamental pond fish, koi were discovered quite by accident as coloured genetic mutations (accidents of nature) from native black carp which the farmers had introduced from Asia.

The carp were introduced into the paddy fields to supplement their bland diet. However, the carp bred naturally, producing many thousands of offspring, some of which were not black like their parents but pale yellow.

Over many generations, instead of being eaten, the cherished pale yellow koi were crossed with other similarly coloured genetic freaks. Different regions of Japan are reputed to have given rise to different koi varieties that are found in our ponds today.

Koi are produced in several key areas of the world where the climate is suitable for farming. Japanese koi offer the best quality or ‘pedigree’ with deep red pigmentation and clean skin in metallic varieties. Koi from Israel offer excellent value for money and at times, can be difficult to separate from the consistently excellent Japanese fish.

The vast array of patterns and colours available today seems unlimited as the pattern of each koi is unique. It is the knowledge that a truly excellent koi is irreplaceable that makes the top end of the market out of bounds financially for the majority of people. Nevertheless, one of the major attractions of keeping koi is that there will always be a wide selection of koi to choose from irrespective of your budget, be it a few pounds of spending money or a month’s salary.

Did you know?

Only approximately 1% of a typical spawn in Japan reaches the market. The other 99% do not make it as they are not  considered to have reached the desired grade and are culled as early as 3 weeks old.

Koi have inherited many similar characteristics from their distant carp ancestors being omnivorous scavengers (eating both plant and animal material), often being referred to as the pig of the pond. On close inspection of the koi anatomy, it’s downturned mouth or ‘snout’ and barbels make it perfect for rooting around on the pond bottom. In fact, koi would prefer to feed from the pond bottom rather than the surface but this would deny us the opportunity of admiring them closely as they feed on floating pellets.

Koi have also retained the endearing quality of becoming very tame, often being so friendly as to take food from the hand.

Essentially a warm water fish, koi prefer a stable pond temperature but can tolerate a wide range throughout the year from just above freezing to 30oC, preferring and spawning in the mid 20oC range.

One of the most significant differences between koi and their robust ‘wild’ carp ancestors is that koi are not as hardy as their ancestors. As koi are easily stressed and susceptible to disease so a stable, well-filtered and deep pond is essential to keep the koi in tip-top health.

Kill blanketweed and string algae.