Koi Pond Maintenance: A Complete Seasonal Guide for UK Pond Owners

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Koi are among the most rewarding fish you can keep — and among the most demanding. A well-maintained koi pond is a genuinely stunning feature. A neglected one is an expensive disaster. This guide covers what you need to do across each season to keep your koi healthy and your pond looking its best throughout the year.

Spring: Wake Up the Pond

Spring is the most critical time of year for koi pond owners. As water temperatures rise above 10°C, your fish become more active, start eating again, and their immune systems come back online after winter dormancy — but they're also at their most vulnerable to disease during this transition.

What to Do

• Start testing water parameters weekly as temperatures rise
• Reintroduce feeding gradually — start with easily digestible wheat germ food
• Check all equipment thoroughly after winter — pumps, filters, UV sterilisers
• Clean out any debris that has accumulated over winter
• Check your fish carefully for signs of winter illness — ulcers, fin damage, unusual behaviour
• Get your UV steriliser running — spring is when algae blooms begin

Summer: Peak Season Management

Summer is when your pond looks its best and your koi are at their most active. It's also when problems can escalate fastest — warmer water holds less oxygen, algae grows aggressively, and fish are eating and producing waste at their maximum rate.

What to Do

• Test water parameters weekly — ammonia, nitrite, nitrate, pH, and dissolved oxygen
• Feed daily, adjusting quantity to what fish consume in 5 minutes
• Keep on top of algae — string algae in particular
• Ensure adequate aeration — add an air pump if dissolved oxygen drops
• Keep on top of water changes — 10–15% weekly in hot weather
• Net the pond if herons are active in your area

Autumn: Prepare for Winter

Autumn is about preparation. The work you do now determines how your koi come through winter.

• Net the pond before leaves start falling — decomposing leaves crash water quality fast
• Gradually reduce feeding as temperatures drop below 15°C
• Stop feeding entirely below 10°C — koi cannot digest food properly at low temperatures
• Clean filters thoroughly before winter
• Check pond depth — koi need at least 1.2 metres of depth to overwinter safely in the UK
• Service all equipment before it gets cold

Winter: Minimal Intervention

Koi effectively hibernate in winter. Their metabolism slows dramatically and they require very little from you — but there are some critical things to monitor.

• Do not feed
• Ensure the pond doesn't completely ice over — trapped gases beneath the ice can be fatal. Use a pond heater or de-icer to maintain an opening
• Never break ice by hitting it — the shockwave can kill fish. Use hot water to melt a hole gently
• Keep the pump and filter running at a reduced rate to maintain water movement

Year-Round Essentials

UV Steriliser

Run it from spring through autumn to control green water algae and kill waterborne pathogens. Replace the bulb annually even if it appears to still be working — output degrades before the bulb fails visually.

Filtration

Never turn off your filter, even in winter. The beneficial bacteria need a constant flow of oxygenated water to survive. Clean filter media in pond water only — never tap water, which will kill the bacteria.

Quarantine

Any new fish should be quarantined for a minimum of four weeks before introduction to your main pond. Introducing disease to an established collection is one of the most common and preventable causes of fish loss.

When to Call a Professional

Some situations call for expert input: unexplained fish deaths or illness, persistent water quality problems that won't resolve, ulcers or visible wounds on fish, equipment failure you're unsure how to address, or a new pond setup you want to get right from the start. At Aquatics Simplified, we offer seasonal pond maintenance programmes tailored to your pond size and stocking level.