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How to Winterize a Koi Pond

Introduction: Winterizing Your Koi Pond the Right Way

Koi can survive freezing winters—even in places like Northeast Ohio—but only if the pond is properly winterized. Cold weather slows their metabolism, affects water quality, reduces oxygen levels, and increases the risk of harmful gases becoming trapped beneath ice.

Winterizing a koi pond is about protecting the fish and keeping the pond’s ecosystem stable until spring. This guide explains everything you need to do before the first freeze to ensure your koi stay healthy all winter long.

Clean the Pond Before Temperatures Drop

A clean pond is essential going into winter. As water gets colder, beneficial bacteria slow down and can’t break down debris as effectively.

Remove:

  • Leaves

  • Sludge

  • Twigs

  • Dead plant material

  • Uneaten food

  • Any floating debris

Decaying organic matter releases ammonia and toxic gases under ice. Cleaning now prevents problems later.

For stubborn bottom sludge, use a natural bacterial treatment like Mock Sludge Digester to break down remaining muck before winter sets in.

Switch to Cold Water Koi Food (Then Stop Feeding)

Feeding must follow water temperature—never air temperature. Check this guide for in-depth feeding instructions: “How to Feed Koi Properly”

Feeding transition schedule:

  • Below 60°F: Switch to a wheat germ–based cold-water food

  • Below 50°F: Stop feeding completely

Koi cannot digest food below 50°F. Feeding at low temperatures can cause food to rot in their digestive tract, which can be fatal.

Trim Back & Prepare Pond Plants

Aquatic plants need winter preparation too.

Do the following:

  • Trim hardy plants to a few inches above the crown

  • Remove dead or dying foliage

  • Move potted hardy plants to the deepest part of the pond

  • Bring tropical plants indoors or compost them

Healthy plants reduce debris and improve water quality over the winter months.

Install a Pond Net to Catch Falling Leaves

Falling leaves are a major source of winter pond problems. A simple pond net keeps them out.

Benefits of netting:

  • Reduces sludge accumulation

  • Prevents oxygen depletion

  • Prevents ammonia spikes

  • Saves time during spring cleanouts

Remove the net once all leaves have dropped for the season.

Keep a Hole Open in the Ice (Critical for Fish Survival)

Koi do NOT need the entire pond ice-free—but they must have a hole in the ice so toxic gases can escape.

Best ways to keep a ventilation hole:

✔ Pond De-Icer (Heater)
✔ Aerator / Air Pump placed 1–2 feet below the surface

NEVER do this:

✘ Do NOT smash ice
✘ Do NOT run a waterfall in deep winter

A de-icer + aerator combo keeps koi safe without disturbing their winter dormancy.

Remove and Store the Pond Pump

Your pump will fare much better over time if it is not going through freeze-thaw cycles.

  • Turn off the pump

  • Remove it

  • Clean it

  • Store in a bucket of water where it won’t freeze

The pump should NOT be ran when water can freeze. An ice dam can form at your waterfall and drain the pond under the ice. You won’t notice until the ice finally melts in the spring, revealing a dry pond bed and dead fish.

Reduce Water Movement at Pond Bottom

Koi survive winter by resting in the warmest water near the bottom. DO NOT disturb this area.

Avoid:

  • Deep aeration

  • Bottom-return jets

  • Strong circulation near the base

Let koi remain still and undisturbed.

Monitor Water Quality Throughout Winter

Even during winter, water quality matters.

Test monthly for:

  • pH

  • Ammonia

  • Nitrite

  • KH (carbonate hardness)

A stable, clean winter pond helps koi emerge healthier in spring.

Protect Koi From Winter Predators

Predators like herons still hunt ponds in cold weather.

Use:

  • Netting

  • Decoys

  • Reflective tape

  • Motion-activated deterrents

Koi are slower in cold water, making them easier targets.

Observe Your Koi (Without Disturbing Them)

Healthy overwintering koi should:

  • Sit calmly at the bottom

  • Move slowly

  • Show no signs of gasping or flashing

Do NOT:

❌ Tap ice
❌ Add warm water
❌ Stir or “wake up” koi

Their winter dormancy is natural and healthy.

A Properly Winterized Pond Protects Your Koi

Winter care is about stability, oxygen, and cleanliness. Follow these steps and your koi will stay safe, healthy, and stress-free until spring:

  • Clean the pond

  • Transition food and stop feeding at 50°F

  • Prepare plants

  • Install netting

  • Keep a hole open in the ice

  • Protect pumps

  • Avoid disturbing bottom layers

  • Monitor water quality

  • Deter predators

  • Allow koi to rest

For winter supplies—de-icers, aerators, beneficial bacteria, and pond netting—visit Mock Pools & Ponds, serving Canton, Akron, and Northeast Ohio.

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