Build A Garden Koi Pond

Koi and goldfish swimming in a pond with lily pads on the surface.
By

A Garden Pond Suitable for Koi

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.

Siting and Setting Up a Garden Pond: A Practical Guide to Natural Design

The most successful garden ponds are those that look as though they’ve always been there — shaped by time rather than tools. A well-designed pond should sit comfortably within its surroundings, blending into the garden rather than appearing imposed upon it. Achieving this natural effect requires careful planning around positioning, shape, depth and planting.

Positioning Your Garden Pond

Choosing the right location is one of the most important decisions when building a pond.

In nature, ponds form in low-lying hollows where water naturally collects. In most gardens, however, ground levels are fairly flat, and artificially creating a deep hollow can be problematic — especially if the water table is high. Persistent groundwater seepage can complicate liner installation and long-term stability.

Avoid siting a pond directly beneath trees. Falling leaves increase organic debris, reduce water quality and fuel algae growth. Certain trees — including poplar, oak, elder, willow and yew — may also introduce tannins or toxic compounds into the water. Tree roots can complicate excavation and may later threaten the integrity of the liner.

Similarly, avoid positioning your pond too close to house foundations, boundary walls or underground drains. This makes excavation easier and reduces structural risks.

Near the House or Further Away?

Placing a pond near the house allows it to be enjoyed from indoors and simplifies electrical installation for pumps, filters or lighting. Buildings can also provide shelter from prevailing winds.

However, proximity to the house may limit landscaping options. Spoil from excavation is often used to build a raised rockery or waterfall backdrop. This feature can appear awkward if positioned too close to the house.

A pond located further into the garden often allows for more natural landscaping. It can become a destination feature — a tranquil space designed specifically for relaxation and wildlife observation.

Choosing the Right Pond Shape

Natural ponds rarely have sharp corners or complex outlines. Instead, they feature sweeping curves and gentle transitions.

For a planted garden pond:

  • Avoid tight angles and narrow recesses

  • Keep edges smooth and flowing

  • Include gradual shelving where appropriate

Simple shapes not only look more natural but are easier to line with flexible pond liners and reduce areas of stagnant water.

Pond Depth and Structure

A planted pond should include varied depth zones:

  • Marginal shelves: At least 9 inches below water level to support aquatic baskets. Shelves need not run continuously around the perimeter.

  • Central depth: A minimum of 2 feet is recommended.

Greater depth improves winter protection, reducing the risk of complete freezing. Deeper ponds are also more thermally stable — they resist rapid heating and cooling. Larger volumes dilute pollutants more effectively, creating a healthier environment for fish. Quite simply, dilution is the solution to pollution.

Stocking a Planted Garden Pond

Most coldwater ornamental fish species are compatible with each other, but not all suit a planted pond environment.

Koi and ghost carp are generally unsuitable. Their constant rooting behaviour disrupts planting baskets and clouds the water. They also grow large and require more space and filtration.

Better choices for planted ponds include:

  • Standard goldfish

  • Comets

  • Shubunkins

  • Golden orfe

  • Tench

Stocking guidelines are best calculated based on pond surface area rather than volume. As a rough guide, allow approximately 3 inches of fish (excluding tail) per square foot of clear surface area. Always stock gradually and monitor water quality carefully.

Remember: ponds are most stressed during the warmest summer conditions, so stock conservatively.

Essential Pond Equipment

One of the advantages of a well-balanced planted pond is that it requires relatively little equipment.

Pond Construction Materials

The two most common options are:

  • Preformed fibreglass ponds

  • Flexible PVC or rubber liners

Preformed ponds are quick to install but limit design flexibility and can be more expensive.

Flexible liners offer complete creative control and are typically guaranteed for up to 25 years. To calculate liner size, measure the maximum length and width of the excavation and add twice the maximum depth to each dimension.

Pond Pumps and Filtration

Lightly stocked wildlife or planted ponds may function naturally without heavy filtration.

If fish stocking increases, install a pump and filter system. The pump should circulate the entire pond volume once every two hours. For lightly stocked ponds, a small internal foam filter may be sufficient. Larger systems may require an external biological filter, which can also feed a waterfall feature.

A balanced partnership between plants, filtration and sensible stocking will produce the best long-term results.

Final Thoughts

Creating a natural-looking garden pond is about restraint and thoughtful design. Choose the right location, shape it simply, provide adequate depth and stock it wisely. With careful planning, your pond will settle seamlessly into the landscape and provide years of enjoyment for both fish and gardener alike.

Share This Article

Related Articles