Dive Into the World of Koi: The Complete Guide

Welcome to the World of Koi Ponds

There's something undeniably calming about the gentle ripple of water, the glimmer of colourful koi gliding beneath the surface, and the soft rustle of pond plants in the breeze.

Whether you're a seasoned enthusiast or just beginning to explore the beauty of backyard water features, this blog is your guide to creating and maintaining a thriving koi pond.

From design tips to fish care, we’ll dive into everything you need to know to turn your outdoor space into a peaceful aquatic haven.

Why Build a Koi Pond?

· Enhances landscaping - A koi pond brings natural beauty and tranquillity to your home, serving as a stunning centrepiece in any garden

· Promotes relaxation - The gentle sound of flowing water, paired with the soothing sight of koi swimming gracefully, creates a calming atmosphere. Many people find koi ponds to be therapeutic spaces perfect for meditation, unwinding after work, or simply enjoying a quiet moment outside.

· Supports aquatic life - A well-maintained koi pond fosters a thriving mini-ecosystem. Beyond koi, it can support aquatic plants like water lilies, frogs, dragonflies, and birds that visit or make the pond their home.

· Adds colour and movement - Koi are known for their vibrant colours and patterns. Their constant motion adds energy and life to the garden!

· Customizable design - Koi ponds can be built to suit virtually any space—from compact features to large backyard retreats. You can choose the shape, depth, filtration system, lighting, and landscaping to match your vision and budget.

· Family-friendly - Koi ponds offer educational value for children and a unique way to connect with nature. Feeding koi can be a fun daily activity, and children often enjoy learning about the fish and how ecosystems work.

· Low-maintenance ecosystem - Once established with the right balance of plants, filtration, and fish, koi ponds can become stable ecosystems that require minimal interference.

· Year-round interest - Koi ponds offer year-round beauty—from blooming life in spring and summer to colourful reflections in fall, and active koi in mild winters

Koi Carp And Their Needs

Koi are far more than ornamental pond dwellers—they are living symbols of serenity, peace, and perseverance. With their graceful movements and mesmerizing colours, koi have long been revered in many cultures, especially in Japan, where they are seen as emblems of strength and tranquillity.

Behind their vibrant patterns and colours lies a delicate biology that requires thoughtful care. These peaceful creatures thrive in clean, well-maintained environments and are sensitive to changes in water quality and temperature. Caring for koi is not just about keeping fish. It's a practice in mindfulness, patience, and respect for life.

To keep koi is to invite calm into one’s space, and to honour the balance between beauty and responsibility. Dive deeper into the different types of koi by exploring our blogs all about Koi varieties [here].

For a more detailed account of some of the most popular koi varieties learn all about Kohaku, Sanke and Showa, as well as Ogon and Utsuri koi.

Designing The Pond

Design goes beyond just shape. It's the careful balance of size, water quality, filtration, fish, and plants working in harmony. A truly successful koi pond is not only visually stunning but also easy to maintain. That harmony is achieved when form follows biology.

The size of your pond affects more than just how many koi you can keep. It also influences how easy it is to maintain, how the water behaves, and what kind of equipment you'll need to support a healthy environment.

Pond Size Tiers & Recommendations for Keeping Koi

Small Ponds (Under 5,000 litres)

- Recommended Koi: Up to 10 small koi (under 8”) to allow room for growth
- Care Level: High maintenance – water quality can fluctuate quickly
- Filtration Needs: High-efficiency filtration and frequent monitoring using a suitable water test kit. For ponds under 3,000 we would recommend the Pondxpert Spinclean 12,000 filter and pump
- Best For: Beginners with limited space or those planning to upgrade later
- Note: More suited for goldfish or low amounts of koi or as a starter pond rather than long-term koi care

Medium Ponds (5,000–14,000 litres)

- Recommended Koi: 10–30 medium-sized koi
- Care Level: Moderate – easier to maintain stable water conditions
- Filtration Needs: Strong biological filtration such as the Nexus 220+ Filter System. For ponds under 8,000 litres, the EazyPod can be a suitable filtration option, depending on the number and size of fish stocked.
- Best For: Hobbyists looking for a balance between space and manageability
- Note: Good balance of aesthetics and practicality

Large Ponds (15,000–30,000 litres)

- Recommended Koi: 30-60+ koi depending on size
- Care Level: Easier to maintain stable ecosystem but more expensive to build
- Filtration Needs: Advanced filtration systems such as the Nexus 320+ Filter or a Drum Filter, possibly with additional UV Systems, Bead Filters, bottom drains and Spindrifters to best maintain water quality and clarity.
- Best For: Dedicated koi keepers and serious hobbyists
- Note: More room for koi to grow to full size and display natural behaviour

Extra-Large Ponds (30,000+ litres)

- Recommended Koi: 60 -100+ koi, of varying sizes to allow plenty of space for natural growth
- Care Level: Lower daily fluctuation risks, but requires planning and investment
- Filtration Needs: Professional-grade equipment and water management systems. Multiple advanced filtration systems to cope with the sheer volume of water and waste processing. For greater clarity, additional UV Systems may be required as well as water polishing equipment such as a Bead Filter.
- Best For: Koi collectors, breeders, or those creating a centrepiece water feature
- Note: Offers the most natural and stress-free environment for koi

As a general guideline, the larger the pond, the more stable and resilient the ecosystem tends to be. Choosing the right equipment for your pond’s size and stocking levels can prevent many issues down the line.

When designing your pond, don’t just focus on what’s visible. Prioritise what’s hidden beneath the surface: filtration, circulation, and aeration. These elements are the foundation of a truly sustainable pond.

If you need guidance or support with building your pond, feel free to reach out. We’re here to help you start off right. We also provide professional pond building and maintenance services for ponds of all sizes.

Ideal Placement

Designing a pond is one thing. Choosing the right location is another. Where you place it affects how easy it is to maintain, how well your fish will thrive, and how much you'll enjoy it day to day. Keep reading to explore key landscaping tips and common pitfalls to avoid when deciding where and how to build your pond.

Best Practices

Choose a level site
–A flat or gently sloping area is easier to build on and helps avoid drainage issues.
Ensure good visibility
– Place the pond where it can be easily seen and enjoyed, such as near patios, windows, or walkways.
Allow access for maintenance
– Leave enough space around the pond for equipment, cleaning, and seasonal care.
Consider sunlight
– Aim for 4–6 hours of sunlight per day; partial shade helps control algae and keeps water temperature stable.
Plan for natural drainage
– Situate the pond away from areas where rainwater runoff could carry debris, fertilizers, or contaminants.
Use surrounding landscaping to enhance
– Incorporate plants, rocks, and edging that complement the pond and help it blend naturally into the environment.
Think about elevation
– Slight elevation can protect from floodwater and help with visual impact.
Allow for future expansion
– Leave room in your design if you might want to increase pond size or add features later.

What to Avoid

Avoid low-lying, boggy areas
– These can collect runoff and cause poor water quality or flooding.
Don’t build directly under trees
– Falling leaves and roots can cause ongoing maintenance issues and structural damage.
Avoid full sun all day
– This can overheat the water and encourage excessive algae growth.
Don’t place too close to structures
– Foundations, fences, or buried utility lines can interfere with pond construction and access.
Avoid windy, exposed areas
– Wind can cool the water too much, increase evaporation, and blow debris into the pond.
Don’t overlook electrical access
– You’ll need safe, nearby power sources for pumps, filters, lighting, and other equipment.
Avoid cramped corners
- Tight spaces limit both design options and your enjoyment of the pond.

Technical Considerations for Building & Maintaining a Pond

· Pond Size & Depth

Ensure adequate depth (usually at least 1 meter/3 feet) to provide stable water temperature and protection for fish during extreme weather.

· Filtration System

Choose the right mechanical and biological filtration based on pond size and fish load to maintain water clarity and quality.

· Water Circulation

Proper circulation prevents stagnation, distributes oxygen, and supports beneficial bacteria.

· Aeration

Incorporate air pumps or waterfalls to increase oxygen levels, especially in warmer months.

· Water Source & Quality

Use clean water free of harmful chemicals, remember to always use a dechlorinator treatment on any hose water you may use. Test regularly for pH, ammonia, nitrites, and nitrates, KH and GH by using a pond water test kit , to catch and address problems early.

· Pump Capacity

Select a pump that can circulate the entire pond volume at least once every 1-2 hours.

· UV Clarifiers

Consider adding UV systems to control algae and harmful pathogens.

· Electrical Safety

Use ground-fault circuit interrupters (GFCI) and waterproof connections to prevent electrical hazards.

· Winterization

Plan for cold weather with heaters, de-icers, or aerators to keep part of the pond surface ice-free.

· Overflow & Drainage

Design overflow systems to handle heavy rain without flooding or pond damage.

· Liner Type & Installation

Choose durable rubber pond liners (EPDM) and ensure proper installation and sufficient underlay to prevent future leaks.

· Fish Stocking Density

Avoid overcrowding; maintain appropriate fish-to-water ratios to reduce stress and disease risk.

Excavation And Materials

The quality of excavation and the materials used in constructing a pond are vital to the long-term health of your fish. Without a solid foundation, a pond cannot achieve its full potential.

Steps for Building a Pond

1.Marking the Layout

Determine the pond’s desired size and shape.
Use stakes and string or spray paint to outline the perimeter on the ground.
Ensure the layout fits well within the available space and considers sunlight exposure and drainage.

2. Planning Depth Zones

Designate shallow, medium, and deep zones for aquatic plants and fish habitat.
Typically, include:

- Shallow zone (0–1 foot) for marginal plants.

- Intermediate zone (1–3 feet) for submerged plants.

- Deep zone (3+ feet) to provide refuge for fish during temperature changes or winter.

3. Creating Slopes

Plan gentle slopes for the pond edges to prevent erosion and provide safe access for wildlife.
Ideal slope ratio: about 3:1 (horizontal to vertical) or gentler, especially in shallow zones.
Steeper slopes can be used in deeper areas if lined properly.

4. Removal of Sharp Objects

Carefully excavate and remove rocks, sharp stones, tree stumps, roots, and debris.
Check thoroughly to avoid puncturing pond liners or harming aquatic life later.
Use appropriate tools (shovels, rakes, root saws) to clear the area completely.

5. Choosing Pond Material

Pond liners are commonly used to retain water and prevent seepage. Main types include:

PVC liners: Flexible, affordable, and easy to install; suitable for small to medium ponds. Best for ponds with very few fish or wildlife ponds.
EPDM rubber liners: Durable, UV resistant, flexible, and longer-lasting; ideal for larger ponds.
Butyl liners: Very durable but more expensive; often used for commercial or large ponds.
Concrete: Permanent and durable but costly and requires more expertise; used in formal or ornamental ponds. These can then be sealed with a suitable pond paint or sealant such as G4.
Fibreglass: Fibreglass: Extremely durable and long-lasting with a smooth, non-porous finish that resists algae growth. Prefabricated fibreglass ponds are available in various shapes and sizes and are easy to install, while custom-built fibreglass linings are often used for high-end or formal ponds. Installation requires a solid base (often concrete) and professional application for custom builds. Fibreglass is more expensive upfront but offers minimal maintenance and a clean aesthetic.

6. Preparing the Base for Liner Installation

Smooth out the excavated pond bottom and sides; remove any sharp rocks, roots, or debris that could damage the liner.
Compact the soil lightly to create a stable, even surface without sharp projections.

7. Underlay Installation

Use a protective underlay (also called a liner underlay or geotextile fabric) beneath the pond liner to prevent punctures and extend liner life.

Materials for underlay include:

Felt underlay (natural or synthetic felt pads).
Geotextile fabric (durable and breathable).
Sand or fine soil layer (if a fabric underlay is not used, a thick layer of fine sand can cushion the liner).

Ensure the underlay fully covers the entire pond base and sides, with some overlap at the edges.

Liner Installation Tips

Allow the liner to loosely conform to the pond’s shape, avoiding tension or folds.
Leave extra liner material around the edges to secure it with rocks or edging materials.
Once installed, fill the pond slowly with water to help the liner settle into place.
Keep extra liner material for future repairs.

Pro Tips & Tricks

Don’t cut the liner until the pond is full. This lets it stretch and fit the true shape perfectly.
For crystal clear water from day one, install bottom drains during construction.
Use a garden hose to smooth out liner wrinkles as you fill the pond slowly.
Build gentle shelves inside the pond for plants and habitat zones.
If you have a square or rectangular pond – use a box-welded liner for easy installation.

Filtration

Why Clean Water Means Better Koi

Healthy water reduces stress, improving koi immune systems.
Clear water allows koi to display vibrant colours and natural behaviour.
Fewer toxins and pollutants lead to better growth and longevity.
Clean water shows off your koi in all their glory.

Why Filtration Is Critical and What It Does

Removes physical debris like uneaten food and waste.
Breaks down harmful chemicals (like ammonia and nitrites) through biological filtration.
Maintains oxygen levels and water clarity.
Supports a balanced aquatic environment for koi and plants.

Risks of Unfiltered Water

Build-up of toxic substances causing fish stress or death.
Cloudy, murky water reduces oxygen and sunlight penetration.
Increased risk of algae blooms and disease outbreaks.
Poor water quality leads to foul odours and unsightly ponds.

Key Benefits of Filtration

Keeps water clear and odour-free.
Supports healthy fish growth and behaviour.
Creates a stable, balanced ecosystem.
Reduces pond maintenance frequency and effort.

Explore our complete selection of filtration systems available both in-store and online. Whether you’re looking for advice on choosing the right filter or want to learn more about maintaining your current setup, the knowledgeable team at Complete Koi is here to help.

Contact us today for expert guidance tailored to your pond’s needs and find the perfect filtration solution for your koi.

Additional Systems For Perfect Water

Filtration is undoubtedly the most essential component of any pond setup. However, when combined with other supporting systems such as aeration, UV clarifiers, and regular maintenance, it can truly elevate your pond, transforming it into a healthy, vibrant oasis. Together, these elements create the perfect environment for your koi and aquatic plants to thrive.

Pond Pumps

What it does: Circulates water throughout the pond and filtration system.
Benefits:

– Prevents stagnation and promotes oxygenation.

– Supports filtration efficiency by moving water through filters.

– Helps maintain water clarity and health.

How it works:

– Electric motor drives an impeller that moves water through pipes or tubing.

– Flow rate depends on pump size and pond volume.

Recommendation:

– Choose a pump rated to circulate the entire pond volume at least once every 1-2 hours , we find the Hailea pond pump range to be a great powerful yet budget friendly option.

UV Systems

What it does: Uses ultraviolet light to kill algae, bacteria, and parasites in pond water.
Benefits:

– Controls green water (algae bloom) and improves water clarity.

– Reduces harmful bacterial organisms.

– Supports overall pond health without chemicals.

How it works:

– Water passes through a chamber containing a UV bulb; UV light sterilizes microorganisms. Remember you need to replace the bulb every year for continual results.

Recommendation:

– Choose a UV clarifier appropriate for your pond size. Remember, when it comes to UV systems, larger units often deliver better results. We highly recommend Evolution Aqua UV units for optimal performance and reliability.

Air Pumps

What it does: Supplies air to the pond, increasing oxygen levels.
Benefits:

– Improves oxygen availability for fish and beneficial bacteria.

– Helps prevent stratification and supports aerobic filtration.

– Can power decorative features like fountains or bubblers.

How it works:

– Compresses and pushes air through tubing into diffusers or air stones underwater.

Recommendation:

– Use an air pump appropriate for pond size with adjustable airflow (e.g. PondXpert ElectroAir 2400- this comes with all fittings required for easy setup).

Skimmers

What it does: Removes floating debris (leaves, twigs) from the pond surface before it sinks.
Benefits:

– Keeps pond surface clean and improves appearance.

– Reduces organic matter entering the pond, lowering maintenance.

– Enhances filtration efficiency by reducing debris load.

How it works:

– Water is drawn into the skimmer, trapping debris in a basket for easy removal.

Recommendation:

– Choose a skimmer sized for your pond volume and expected debris load and decide whether you would like a fixed skimmer e.g. Certiken skimmer (better on larger ponds) or a floating skimmer – more suited to small ponds.

Koi Feeding And Care

Koi don't eat the same way all year. Their metabolism changes with water temperature and environmental conditions. Feeding in tune with the seasons supports their health, digestion, and overall well-being.

Spring Diet (Water Temps: 10–18°C)

Start slow: Feed easily digestible, wheatgerm-based food as koi become active, our Staple Diet food is a great choice for this time of year.
Feed small amounts: Once daily to avoid overfeeding while metabolism is still slow.
Monitor temperature: Increase feeding as water warms above 12°C (54°F).
Boost immunity: Add probiotic/enzyme-rich additive to support health post-winter.
Supplement: Occasional treats like orange slices or spinach can help boost energy.

Summer Diet (Water Temps: 18–28°C)

High-protein growth food: Support energy, growth, and colour development.
Feed up to 3 times daily: More frequent feeding is fine as digestion is at its peak. Just ensure regular water monitoring to ensure you filtration can cope with the increased waste load.
Variety is key: Mix pellets with treats like shrimp, peas, watermelon, or koi-safe veggies.
Enhance colour: Include colour-enhancing food such as our Koi Supergrowth Food as well as high protein treats through warmer months
Stay clean: Remove uneaten food to maintain water quality.

Autumn Diet (Water Temps: 15–10°C )

Gradually reduce feeding: As temperatures drop, koi metabolism slows down.
Switch to wheatgerm-based food: Easier to digest as their system begins to slow.
Feed once daily or every other day: Only what they can eat in a few minutes.
Monitor temperature closely: Stop feeding when water consistently falls below 10°C. Invest in a pond thermometer to ensure you can catch any sudden changes in the weather.
Prepare for winter: A healthy, low-fat autumn diet helps koi overwinter well.

Winter Diet (Water Temps: Below 10°C)

Reduce then stop feeding entirely: Koi enter a state of dormancy and cannot digest food properly. They can digest Wheatgerm in temperatures as low as 8°C, however most koi will refuse feeding entirely at this point.
Avoid feeding even if they seem active: Digestion slows too much, and food can rot in their gut.
Monitor water temp regularly: Use a pond thermometer to know when to resume feeding in spring.
Support the pond: Keep water well-oxygenated and clear of ice if possible (e.g., with an air pump or pond heater).
Watch your koi: Healthy koi will rest near the bottom and remain mostly still during winter.

It is important to note these guidelines are for a non–heated pond in the UK. If you’re pond is heated, then you can follow the Spring and Summer diet plan year-round!

As a general guideline, koi can be fed once per day throughout the year (excluding winter) but only as much as they can consume within a couple of minutes. Any uneaten food should be removed promptly to maintain water quality.

If leftovers are common, you’re likely overfeeding. Simply reduce the amount to match what your koi can comfortably finish in that short window.

During warmer months, you can increase feeding frequency by splitting the daily portion into smaller meals throughout the day—this supports better digestion and growth.

Signs Of Discomfort

Feeding time is more than just an opportunity to nourish your koi—it's the perfect moment to observe them closely. By watching your fish during feeding, you can spot early signs of distress, changes in behaviour, or physical health issues without the need to catch or handle them.

What to Watch for During Feeding – and What It Might Mean

Unusual swimming patterns

– May signal swim bladder issues, stress, or neurological problems.

Clamped fins (fins held close to the body)

– Often a sign of stress, poor water quality, or early illness.

Surface gasping

– Could indicate low oxygen levels, gill parasites, or poor water conditions.

Sores or ulcers on the body

– Possible bacterial infections, physical injury, or parasitic damage.

Discoloration or faded colours

– Might be due to stress, poor nutrition, or early disease symptoms.

Visible parasites (e.g., white spots, anchor worms)

– Indicates a parasitic infection that needs prompt treatment.

Lethargy or isolation from the group

– Can point to internal illness, infection, or high stress levels.

If you’re ever unsure about your koi’s health, don’t hesitate to reach out. We’re always here and happy to offer advice and support to help keep your fish healthy and thriving.

Pond Maintenance

Pond maintenance is a crucial part of caring for your koi. Keeping your water clean and balanced creates the ideal environment for healthy, happy fish and helps prevent many common problems. A well-maintained pond supports clear water, strong filtration, and vibrant aquatic life.

To help you stay on track, here’s our handy maintenance checklist for regular pond care:

Pond Maintenance Checklist

Weekly:

Remove debris and floating leaves from pond surface.
Check and clean skimmer baskets and pump intakes.
Monitor water clarity and koi behaviour.
Top up water lost to evaporation.

Monthly:

Inspect and clean pond filters and UV clarifiers.
Test water quality (pH, ammonia, nitrite, nitrate).
Check pond equipment (pumps, aerators, skimmers) for proper function.

Seasonal Pond Maintenance:

Remove debris and vacuum sludge (Autumn & Spring).
Clean pumps, impellers, and air stones.

Trim plants and check liner for damage.
Thoroughly clean filter media – in water from the pond
UV systems- clean the quartz sleeve to prevent build up of grime (changing out the bulb annually in Spring)

Partial Water Changes

Performing partial water changes is an important part of maintaining a healthy koi pond. It helps dilute toxins, refreshes minerals, and improves overall water quality without stressing your fish.

Frequency: For smaller pond systems, aim to change 10–20% of the water every 4 weeks. Larger ponds tend to be more stable and may not require changes as frequently, but regular partial water changes are still important, frequency should be adjusted based on pond size, fish load, and overall water quality.
Use dechlorinated water: Always treat tap water to remove chlorine and chloramines before adding it to the pond.
Match water temperature: Add new water gradually and make sure it's close in temperature to the pond to prevent shocking your koi. Avoid doing water changes during or just before sudden or extreme weather conditions.
Avoid full water changes: Completely draining a pond can disrupt beneficial bacteria and stress fish.
Monitor water parameters: Test for ammonia, nitrite, nitrate, and pH regularly to decide if more frequent changes are needed.
Clean before changing: Remove debris and sludge to reduce the amount of dirty water reintroduced. Sludge Bombs can be routinely added to the pond to aid with the breakdown of sludge and detritus at the bottom of the pond.

Regular partial water changes keep your pond environment balanced, supporting healthy koi and clear water.

Common Mistakes To Avoid When Keeping Koi

Keeping koi healthy and thriving requires more than just feeding and enjoying their beauty. Avoiding these common mistakes can save you stress, money, and—most importantly—your fish.

1. Overcrowding

Why it’s a problem: Too many fish create excess waste, which overwhelms the filtration system and reduces oxygen levels. It increases stress, competition for food, and risk of disease.

How to fix: Stock conservatively—allow enough space for smaller fish to grow. Rehome surplus fish or increase pond size and filtration to match your stock.

2. Undersized Filtration

The downside: An inadequate filter won’t keep up with the waste load, leading to poor water quality, algae blooms, and stressed or sick fish.

How to fix: Upgrade to a filter rated for at least 4x your pond volume—more if heavily stocked. Include mechanical, biological, and UV filtration for best results.

3. Poor Oxygenation

What can go wrong: Low oxygen can cause gasping at the surface, lethargy, or even fish loss—especially in hot weather or at night when oxygen naturally dips.

How to fix: Add air pumps, surface aerators, or waterfalls to improve circulation and oxygen levels. Ensure water movement across the entire pond, not just one spot.

4. Incorrect Feeding

What to watch out for: Overfeeding, feeding during cold temperatures, or using the wrong food type can cause poor water quality and digestion issues.

How to feed like a pro: Feed only what koi can consume in 2–3 minutes. Adjust frequency and food type with the seasons. Use high-quality food suited to temperature and koi size.

5. Inconsistency

Why it’s harmful: Sudden changes in water chemistry, feeding habits, or cleaning routines can stress koi and disrupt their environment.

Tips to stay consistent:

Keep a pond journal.
Track water temperature, pH, fish behaviour, and any maintenance done.
Stick to regular testing and feeding schedules.
Avoid changing all filter media at once. It is best to clean it or replace it in stages to preserve beneficial bacteria.

Pro Tip: A little routine goes a long way in koi keeping. By monitoring, maintaining, and adjusting gradually, you’ll help your koi thrive in a stable and stress-free environment.

Need a Hand with Your Pond? We’ve Got You Covered!

Need help getting your pond setup just right? Whether you're upgrading filtration, adjusting feeding routines, or tackling water quality, our team at Complete Koi is here to help.

Visit us in-store or online today for expert advice, top-quality equipment, and everything you need to keep your koi healthy and thriving.

For more information or for advice on your pond, check out the rest of our blogs…

Understanding Your Water Parameters for a Healthy Pond Ecosystem | All About Goldfish: Your Guide to a Happy, Healthy Pond | Pond Plants: How to Choose the Right Ones and Their Benefits for Your Pond