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To take care of your koi pond in winter, stop feeding once water temperatures fall below 45°F, maintain oxygen levels with an aerator, and keep a small hole open in the ice using a de-icer. Clean debris, trim plants, and protect your equipment before freezing weather arrives.
If you live in the Midwest, you know winter can be brutal. When ponds freeze, oxygen levels drop, harmful gases build up, and koi enter a dormant state near the bottom. Taking a few simple precautions before and during winter helps your fish stay healthy until spring.
Midwest Ponds recommends preparing early — before the first hard frost — to keep your pond running smoothly and your koi comfortable.
As water cools, a koi’s metabolism slows dramatically. Once temperatures fall below 45°F (7°C), they can no longer digest food properly. Continue feeding only a wheat-germ formula until water reaches that mark, then stop completely.
Feeding during cold months leads to waste buildup and harmful ammonia spikes under ice. Always monitor with a pond thermometer to know when to taper off.
Shop Pond Thermometers and Koi Food to prepare for fall and winter feeding changes.
A key to koi survival through winter is oxygen circulation. When a pond freezes over, gas exchange stops. Installing a pond aerator or bubbler ensures consistent oxygen flow.
Place the diffuser at mid-depth instead of the bottom — this keeps the lower water layer warmer for resting koi. Continuous aeration also helps prevent ice from sealing the surface completely.
Explore Pond Aeration Systems and Bubblers from Midwest Ponds.
Even with good aeration, a de-icer is essential during harsh Midwest winters. It maintains a small hole (12–18 inches) in the surface ice to allow gases to escape. Without it, carbon dioxide and methane build up, suffocating fish.
Use a floating de-icer or Kasco de-icer designed for continuous outdoor use. Never hit or break ice manually — vibrations can harm or kill koi.
Check out Kasco De-icers for reliable cold-weather performance.
Leaves, dead plants, and organic waste decompose underwater and deplete oxygen. Before the pond freezes, remove debris with a net or pond vacuum. Add cold-water bacteria to break down leftover muck naturally.
Regular fall cleanup keeps the water clearer and prevents it from going stale under the ice.
Find Water Treatments and Beneficial Bacteria for healthy winter ponds.
Healthy pond plants are part of the ecosystem, but dying vegetation can harm water quality. Cut back hardy plants and remove any tropical varieties that won’t survive frost. Overwinter tropicals indoors if possible.
Hardy lilies can stay submerged if the pond is over two feet deep; otherwise, store them in water-filled containers indoors. Covering your pond with a fine pond net helps stop new debris from falling in.
For deep ponds (2½ feet or more), you can keep the pump running at mid-level to maintain gentle circulation. In shallow ponds, remove the pump to prevent pulling warmer water from the bottom, which can chill your koi.
If you take the pump out, store it in a bucket of water in a frost-free location to keep internal seals moist. Always inspect hoses and fittings before reinstalling in the spring.
See Pond Pumps and Winter Accessories on Midwest Ponds.
When temperatures drop below zero for extended periods, add a layer of protection. Floating covers, foam boards, or greenhouse-style domes help trap heat and block freezing winds. Even partial coverage can prevent deep freezes and protect your aerator and de-icer from ice buildup.
Insulating your pond helps keep water temperatures steady, especially in regions where sub-zero nights are common from December through February.
Once your pond is winterized, check it every week or two. Make sure:
The de-icer hole remains open
The aerator is running
The water level hasn’t dropped
Fish appear calm near the bottom
Quick, gentle checks prevent small problems from turning into major losses.
|
Step |
Action |
Temperature |
|
1 |
Switch to wheat germ food |
50–45°F |
|
2 |
Stop feeding koi |
≤ 45°F |
|
3 |
Install aerator |
Before freeze |
|
4 |
Add de-icer |
At first sign of ice |
|
5 |
Clean debris & add bacteria |
Fall prep |
|
6 |
Trim/remove plants |
Before frost |
|
7 |
Add a cover for insulation |
Below 20°F |
Yes. As long as your pond is at least two feet deep and oxygen is maintained with a de-icer and aerator, koi can hibernate safely through Midwest winters.
Ideally, 24–30 inches. This provides a warm lower layer for koi to rest and prevents complete freezing.
If your pond is deep enough, yes. Run it at mid-level. For shallow ponds, it’s safer to remove the pump entirely.
Stop when the water temperature drops below 45°F. Their metabolism slows, and digestion stops.
No. Salt isn’t necessary unless treating specific fish health issues. It can stress koi during cold periods.
Preparing your koi pond for winter is all about keeping oxygen flowing, water clean, and fish undisturbed. The right combination of aeration, de-icing, and seasonal care protects your koi until spring returns.
Midwest Ponds offers everything you need for cold-weather pond care — aerators, de-icers, water treatments, and accessories designed to perform through Midwest winters.
