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Building a Modern Koi Pond


last updated 3rd April 2019

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How to Build a Modern Koi Pond.

Here we discuss Koi pond construction in some detail, using as an example, a formal stock pond recently constructed for our own business. The pond featured below is an above ground pond utilising modern gravity filtration, and is heated.


It features simple design and construction methods and was required as a show pond for high grade Koi but was also designed to show as much plumbing as possible, so that we could answer as many of the inevitable questions relating to the peculiarities of a modern Koi pond, its construction techniques and maintenance issues.


Before we dig the first spadeful of earth, it is vital with any Koi pond to have a design formulated and written down and on which the construction will be based. Clearly, with so many design possibilities, and with limited experience of Koi pond construction, it can be very difficult to choose the ideal design both in terms of what is really desirable and what will really work well. There is no doubt that the acquisition of a good book on the subject and/or studying the many web sites covering Koi husbandry will help enormously not only with the type of design to be chosen but also with the actual construction techniques employed. Hopefully this web page will help further before you begin your pond building project.  


In our case, the important criteria was to best utilise a small area of available space previously used to house 400 gallon stock ponds for baby Koi adjacent to our main stock ponds. The objectives were:- 1. to construct a show pond which would show off Koi to their best in both one year old and two year old sizes. 2. Be easy to maintain. 3. Incorporate oversized filtration because of the stocking densities envisaged. 4. Incorporate heating for temperature control and 5. Incorporate an Ozone system for ultimate water quality. The final design resulted in a pond approximately 13ft long x 6ft wide and 3ft 6" deep which would stand 3ft out of ground and which we calculated to be around 1700 gallons.  


The ground on which this pond was constructed slopes at an angle of around 10 degrees, but this had been previously levelled as small stock tanks had occupied the area until recently. First we dug out the area to be occupied by the new pond to a depth of around 6" below the pre existing level. Then we added a material called MOT type 1 in order to provide a stable, level and well compacted base on which the concrete base would be laid. Type 1 is use in road construction and is a loose fill aggregate that when machine compacted provides a very firm and stable sub base for a wide variety of construction projects. The pictures show the prepared area, with the bottom drain located on a small pad of concrete, and subsequently levelled and glued in place to 4" pressure pipe which will feed the filter system.

With the bottom drain pipe work in place, the sub base firmly compacted, concrete was mixed and prepared and placed around the bottom drain and the pipe work to prevent any movement when the concrete bas was being placed.



Once our bottom drain pipe work was well secured, we left the supporting concrete to partially cure for 24hrs before we then shuttered the area to be laid to concrete using stout wooden boards and pegs. (see above) The base for this size of pond was designed for 6" of reinforced concrete. 1.5 cu metres of ready mixed concrete was ordered. The mix we used was Gen 3 with plastic fibres already added. The Gen grading system denotes the strength of the concrete and the plastic fibres replace steel mesh reinforcing. Steel is more complex and cumbersome to lay and will rust in concrete unless it is specially protected, especially when used for pond bases.

Plastic fibre reinforcement is cheaper, more flexible, provides a much more consistent form of reinforcement, prevents the concrete from cracking (a common problem with steel reinforcement) and is very very strong. Using Ready mix, it took two people 30 minutes to place and level the base. The concrete was then floated using a steel float to provide a smoother finish. After 4-5 hours, the base was floated again to remove fine tramlines and indentations to leave a smooth and level base (see left) which would be ready for final finishing. In this pond, because we floated the base to a very smooth finish, no render would be required on the base, and therefore the concrete itself was brought right up to the top of the bottom drain lip (see left)



The base was allowed to cure for 48 hours before construction of the walls commenced. As this pond was designed to be only 3ft 6" deep, and to hold around 1,700 gallons of water, the walls were constructed of 100mm dense concrete blocks on edge. The choice of this size of block may come as a surprise because we so often see ponds constructed using 9" hollow blocks backfilled with concrete and reinforced with steel rods. For very large and deep ponds this is excellent practice, but here we are dealing with a much smaller entity. Many people tend to get a little carried away and build ponds which would happily survive a small nuclear explosion and use materials which are inappropriate for the amount of water intended to be contained.

As it is difficult to calculate the stresses involved with water pressure, this is understandable, but can be very costly! The main critical area of strength required in a Koi pond is the base which must be able to support the full weight of water, and must remain intact even in unstable ground conditions. The weight of water in our pond would be just under eight tons when completed, and the water pressure at the base will be less than 3lbs per square inch - not exactly startling. In addition there will be several simple reinforcing techniques used during the construction which will provide ample rigidity and strength to this type of construction - but read on! Please note that the pond return pipes were built in to the walls during construction, and a small overflow was also incorporated in the far wall (just visible in picture right)


The first simple step to reinforcing the structure is to incorporate a simple concrete collar around the outside lower perimeter of the newly constructed block wall to a depth (in this case) of around 8". This provides support and reinforcement where it is needed most, and where the pressure will be greatest, at the bottom of the pond. (see left) In this case this collar also doubled as the foundation for the outer facing brick wall which would provide not only a decorative finish , but further support for the inside block wall. The Outer facing wall when built is 'tied' to the inner wall using steel wall ties built in as construction progresses



Next, we rendered the block walls with a special render mix of sand and cement in the proportions two soft sand to one cement with 1/2kg of plastic reinforcing fibres added to each full mix. This makes the render very, very strong, completely waterproof and resistant to cracking. It is acceptable to use 1 sharp sand: 1 soft sand: 1 part cement, and this will be easier to apply, but more porous when dry. The render is applied in one coat - no scratch coat and top coat - just one coat which is then floated using a steel float. It is easy to see the strength of the render from the very grey colour, this goes almost white when dry.


The render was allowed to cure fully over the next week before we applied the finish. Here we have used Aquacote, a two part epoxy paint, in green to give our pond a very hard wearing, smooth and glossy finish which looks like fibreglass when applied - but with none of the disadvantages (cost and risk of toxicity). The Aquacote is applied by roller once mixed, with corners and awkward areas finished using a brush. Two coats are required. The painted finish cures fully within 24hours and can then be filled with water (and fish!)


Having completed the inside of our pond, we now constructed the outer facing wall and capped this above the brickwork with a limestone capping. This would later be sealed to ensure no lime could be washed into the pond. Remember when choosing brickwork, that many bricks are very porous - typically the machine made ones are worse and are therefore not as hard wearing as we might like, especially when continually exposed to water splashes. Many of the hand made bricks are stronger, because they are less porous and also look much better aesthetically than machine made bricks. There is the usual disadvantage with hand made bricks - cost !


The filter was then positioned and fitted into place. In this case we used a Nexus 200, but the principle is the same whichever filter you choose. (far right). The inlet from the bottom drain is connected to the input side of the filter and it is important to use a slide valve or ball valve between the pond and filter in order to separate the two for cleaning, maintenance, but most importantly to be able to purge the bottom drain pipe work, where dirt will settle out over time. (see left) It is always a good idea to try to be neat and tidy with all the plumbing, here the two drains from the filter have been plumbed together into one manifold and then piped away to waste. This saves pipe work, as well as keeping things neat. (bottom left)

On the outlet side of the filter, be sure to use an appropriately sized ball or slide valve to separate the filter from the pond on the return side. Also fit single union connectors to the pump (see picture left) so that this can be removed for ease of maintenance and in combination with the ball valve fitted here, will ensure that there are no floods when the pump or UV are removed! Note that here the pump, in this case an Oase Aquamax 8000, is positioned on the floor and is tucked away close to the side of the filter so that it is less likely to be stepped on or damaged during access to the rest of the filter plumbing.


For the plumbing in this pond we used PVC class D and E pressure piping throughout. It is very strong, and whilst by no means the only option, is to be recommended. Pressure pipe now costs little more than ordinary solvent weld piping and most pumps, UVs and other pond fittings such as ball and slide valves are made to accept pressure pipe. In the picture, the vertical pipe on the right of the picture leads into the UV from the main filter pump, just out of view here, but pictured clearly above. The water is pumped through the UV, exits the UV on the far left of the picture and is then pumped through a stainless steel heat exchanger. We will be heating this pond using oil as a fuel.

Water then exits at the bottom right of the heat exchanger and here the pipe work splits into two. One branch leads vertically down and then back to the far side of the pond. The shorter branch feeds through the ball valve shown on the far bottom right in the picture to the pond return close to the filter. Note that a ball valve is always required on the shorter branch when using two or more pond returns in order to balance the water flow between the two returns. If this was not fitted, most of the water would exit from the shorter branch. In this installation we have fitted a stainless steel UV. There are a number of good UVs on the market today, and it was claimed that s/s UVs provided more power than conventional UVs - but we now know this not to be the case! Note that, once again, single union connectors have been used between pipe work and the UV itself. This enables easy removal of the unit for bulb replacement and maintenance.

The electrics for the pond were then installed. Here we used an IP rated five way fused switch box to handle the various circuits required from one mains supply. This enables individual control over every element of the installation in safety. The mains supply was connected via an approved RCCB (residual current circuit breaker) unit. The air pump required for the filter was mounted as high as possible and above the water level, close to the filter. In this position, should there be a mains power failure, there is no chance of water syphoning back up the piping into the air pump which would irreparably damage the pump. This model, from the Secoh range is weather proof and very quiet, so needs no external protection from the elements.


And finally, the picture right shows the completed pond, filled with water (and Koi) and happily working as the design intended. The costs to construct this pond, including pipe work, electrics, filter, heating components and all construction materials were in the order of £3000 - so definitely not cheap. This figure does not include labour. Please note that the larger the pond, generally the cheaper the cost per gallon. If you decide to build a concrete pond yourself, supplying your own labour, allow around £2.50 per gallon depending on your design, size and construction materials. If you decide to have the pond built for you, allow a minimum £3.50 - £5.00 per gallon.

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