Koi ponds are one of the most rewarding additions you can make to a garden — but they also come with real questions. Whether you’re thinking about building your first pond or you’ve had fish for years and want to get things right, we’ve pulled together the 12 questions we’re asked most often and answered them honestly. JDM Pond Services has over 20 years of combined experience in aquatic and landscaping trades — the answers below are based on real-world practice, not just textbook theory.

01. How big should a koi pond be?

This is the single most common mistake people make: building a pond that’s too small. Koi are large, active fish that produce a lot of waste, and they need room to grow and behave naturally. As a starting point, we recommend a minimum of 5,000 litres (about 1,100 gallons) for a koi pond. Ideally, for a comfortable, well-balanced environment you’d want 10,000+ litres. The bigger the pond, the more stable the water chemistry, the healthier your fish, and the more forgiving the system when something goes wrong.

🔵 Rule of thumb: Think of your pond as too small today and you’ll be right in two years. Koi can reach 60–90cm in length, and they grow fast in good conditions.

Surface area matters too — a long, wide pond oxygenates better than a deep, narrow one. Aim for a shape that promotes good water circulation and gives fish open water to swim.

02. How deep does a koi pond need to be?

Depth is essential for koi, for two reasons: temperature stability and protection from predators.

Depth	Suitability
Under 90cm (3ft)	Not recommended — overheats in summer, freezes in winter, no heron protection
90cm – 120cm	Minimum for koi; acceptable in milder UK regions with extra care in winter
120cm – 150cm (4–5ft)	Recommended — stable temperature, safer from herons, suitable for most UK climates
150cm+ (5ft+)	Ideal, especially for larger fish or colder areas of the UK

In the UK, a depth of at least 1.2–1.5 metres keeps a layer of unfrozen water at the bottom during winter, which is where your koi will rest. Shallower ponds risk the entire volume freezing, which can be fatal.

03. How many koi can I keep?

There are a few different rules of thumb in the hobby, but the most practical is this: allow 1,000 litres of pond water per koi, assuming good filtration. That’s a conservative guide that keeps your water quality manageable and your fish healthy.

So in a 5,000-litre pond with excellent filtration, you might comfortably keep 4–5 koi — not 10. Overstocking is one of the leading causes of disease, poor water quality, and fish deaths.

🐠 Tip: Start with fewer fish than you think you need. Add more once your filter is mature (usually 6–8 weeks after setting up) and your water parameters are consistently good.

Remember that small koi grow. A 15cm fish bought today could be 60cm+ within a few years. Always stock for the fish you’ll have, not the fish you have now.

04. How much does a koi pond cost in the UK?

Costs vary enormously depending on size, specification, and location — but here’s an honest breakdown to give you a realistic picture:

Type	Approximate Cost
Small starter pond (5,000L, liner, basic filter)	£3,000 – £6,000
Mid-range koi pond (10,000L, fibreglass, quality filtration)	£8,000 – £18,000
Large premium pond (20,000L+, full build, heating, UV, automation)	£20,000 – £50,000+

These figures cover groundworks, lining or fibreglass coating, filtration systems, plumbing and finishing. They don’t include the cost of koi themselves, which can range from £20 for a standard fish to several thousand pounds for a show-quality specimen.

💧 JDM Pond Services offers free, no-obligation quotations for any size of project. Call us on 07540 776504 or email jack@jdmponds.com to get started.

The biggest false economy in pond building is cutting costs on filtration. Spend money here and your pond will be healthy and low-maintenance for years. Cut corners and you’ll spend far more fixing problems.

05. What filtration do I need for a koi pond?

Koi are messy fish. They produce significantly more waste than ornamental goldfish, so filtration is non-negotiable — and it needs to be sized correctly.

A good koi pond filtration system includes three stages:

• Mechanical filtration — removes solid waste (debris, uneaten food, fish waste) before it breaks down.
• Biological filtration — beneficial bacteria convert harmful ammonia and nitrite into less harmful nitrate.
• UV clarifier — kills the single-celled algae that cause green water, keeping your pond crystal clear.

For koi ponds, we often recommend pressure filters or multi-chamber gravity-fed systems, paired with a pump sized to turn over the full pond volume every 1–2 hours. Undersized pumps and filters are the most common reason ponds fail.

🐠 Tip: Always buy a filter rated for at least double your actual fish load. Filter manufacturers rate their products under ideal conditions — real ponds with koi are rarely ideal.

06. How do I keep my koi pond water quality good?

Water quality is everything in a koi pond. Poor water is the root cause of most disease, stress, and fish deaths. The key parameters to monitor regularly are:

• Ammonia — should be 0 ppm. Any ammonia is a red flag.
• Nitrite — should be 0 ppm. Toxic to fish even at low levels.
• Nitrate — keep below 40 ppm with regular water changes.
• pH — koi prefer 7.0–8.5. Stable pH is more important than a perfect reading.
• KH (carbonate hardness) — aim for 100–200 ppm to buffer against pH swings.
• Temperature — koi are comfortable between 15°C and 25°C. Avoid rapid swings.

Test your water at least once a week during the warmer months and once a fortnight in winter. Perform a 10–20% water change every week or two to dilute nitrates and replenish minerals.

💧 JDM Pond Services offers pond water testing and analysis as part of our maintenance packages. We can test your water on-site and advise on treatment if needed.

07. How often should I feed my koi — and with what?

During the active season (spring to autumn), feed your koi 2–4 times a day, giving them only what they can consume in about 5 minutes. Remove any uneaten food promptly, as it will foul the water.

Feed a high-quality wheat germ-based pellet in spring and autumn when water temperatures are cooler (8–12°C), as koi digest this more easily. In summer, a higher-protein growth pellet is appropriate.

As water temperature drops below 10°C, reduce feeding significantly. Below 8°C, stop feeding altogether — koi’s metabolism slows to the point where they cannot digest food properly, and undigested food in the gut can cause serious problems.

🐠 Tip: Overfeeding is one of the fastest ways to ruin water quality. When in doubt, feed less. Koi will not starve between feeds — they will naturally graze on algae and microorganisms in the pond.

08. How do I look after koi in winter?

Koi are remarkably hardy once your pond is set up correctly, but winter does require some attention:

• Stop feeding when water temperature falls consistently below 8–10°C (usually October/November in the Midlands).
• Keep a hole in the ice if the surface freezes — toxic gases from decomposing matter can build up under a sealed surface. Use a pond heater or floating de-icer rather than breaking the ice, which creates shockwaves harmful to fish.
• Reduce pump flow — some ponds benefit from slowing or lifting the pump return to avoid disturbing the warm water layer at the bottom where fish rest.
• Check your filter — beneficial bacteria slow down in cold water, so don’t over-clean the filter in winter or you’ll damage the bacterial colony.
• Watch for illness — fish are more susceptible in spring when their immune systems are still waking up. Watch for unusual behaviour, lethargy, or visible lesions.

💧 We offer winter preparation pond visits across Nottinghamshire, Derbyshire, and Leicestershire — contact us to book a seasonal health check.

09. Can I have plants in a koi pond?

Yes — but with realistic expectations. Koi will dig up, uproot, and eat many pond plants, particularly water lilies and marginals. However, plants do offer real benefits: they absorb nitrates, provide shade, oxygenate the water, and make the pond look beautiful.

The trick is choosing the right plants and protecting them:

• Plant in large, heavy containers with gravel on top to stop koi uprooting them.
• Consider floating plants like water hyacinth or water lettuce in summer — fast-growing and great at pulling nutrients from the water.
• Use marginal planting shelves at the edges where koi are less likely to reach.
• Avoid delicate aquatic plants — koi will treat them as a snack.

A dedicated planting area separated by mesh or a shelf wall can give you the best of both worlds.

10. How do I protect koi from herons and other predators?

Herons are the most serious predator threat to garden koi in the UK. A single heron visit at dawn can empty a pond in minutes. Here are the most effective deterrents:

• Pond netting — the most reliable physical barrier. Fine mesh netting supported above the water surface is very effective.
• Heron decoy — placing a plastic heron at the pond’s edge exploits the bird’s territorial nature. Move it regularly to maintain the effect.
• Trip wire — a single strand of wire 20–30cm high around the pond perimeter is surprisingly effective; herons land at the edge and walk in.
• Motion-activated sprinklers — startle visiting herons and other wildlife effectively.
• Deep water and overhangs — koi instinctively retreat to deeper water when threatened. A pond depth of 1.5m+ with overhanging edges gives them somewhere to hide.

In urban areas, cats and mink can also be a problem. Netting is your most reliable all-round defence.

11. How often does a koi pond need professional servicing?

For most koi pond owners, a professional service once or twice a year — typically in spring and autumn — is ideal. A spring clean clears the sludge that has built up over winter, checks equipment, and sets the pond up for the active season. An autumn visit prepares everything for winter.

Between professional visits, you should be carrying out:

• Weekly water testing
• Regular partial water changes (10–20% fortnightly)
• Monthly filter media rinse (in pond water, not tap water)
• Removal of dead leaves and debris
• Checking pump and UV operation

💧 JDM Pond Services offers flexible maintenance packages across Burton upon Trent, Derby, Nottingham, Mansfield, Grantham and the wider East Midlands. Get in touch to discuss a plan that suits you.

12. What is the best pond liner for a koi pond?

There are two main options for koi pond construction:

EPDM rubber liner — flexible, durable (25+ year lifespan), fish-safe, and adaptable to any shape. It’s the most popular choice for new builds and works well in irregularly shaped ponds. We often use Gordon Low EPDM liner, which is a material we trust.

Fibreglass (GRP) — sprayed or hand-laid into an excavated pond. It’s harder and more permanent, easier to clean, and has a smooth surface with no folds. Fibreglass is our preferred option for larger or premium koi ponds where longevity and hygiene are priorities. JDM Pond Services are fibreglass pond specialists.

Avoid cheap PVC liners for koi ponds — they’re not rated for long-term use with fish, can leach harmful substances, and are prone to UV degradation and puncture.

🐠 Tip: Whatever liner you choose, the groundwork underneath matters just as much. Proper underlay, smooth excavation, and good edge finishing all protect your liner and extend its life significantly.