How to Winterize a Pond
A step-by-step guide to winterizing a pond and protecting koi: when to stop feeding, using a de-icer and aerator, netting for leaves, cleaning debris, pump decisions, and the deep zone.
Winterizing a pond keeps your koi alive through the cold by doing a few simple things right: stop feeding once the water drops below 50 degrees Fahrenheit, keep a hole open in the ice with a de-icer and aerator, net out the leaves, clean the debris before it rots, decide what to do with your pump, and rely on a deep zone of 3 feet or more as a refuge. Koi are cold-blooded and go nearly dormant in winter, so the goal is a calm, oxygenated, gas-free pond, not a warm one. This guide walks each step in order.
Winter Pond Protection Gear
Tetra Pond TetraPond De-Icer (Thermostatic)
Thermostatically controlled floating de-icer that keeps an opening in the ice for gas exchange.
Aquascape Pond Air 2 Aeration Kit
Double-outlet air pump kit to keep oxygen up and help hold the ice hole open all winter.
AlpineReach Koi Pond Netting Kit, 15 x 20 ft
Heavy-duty mesh net with stakes to catch falling leaves and deter herons and predators.
TURBRO Pond De-icer, 400 Watts (Stainless)
Stainless-cased floating pond heater with a long cord and leakage protection for cold climates.
Step 1: Stop feeding below 50 degrees
Koi are cold-blooded, so their metabolism tracks the water temperature. As the water cools into the 50s, slow their feeding and switch to an easily digested wheat-germ food. Once the water drops below 50 degrees Fahrenheit and stays there, stop feeding entirely. Cold koi cannot properly digest food, and anything they eat can rot inside them and cause serious harm. Go by a pond thermometer, not the air temperature, because water holds its warmth longer than the air. Through winter, koi live off the reserves they built up over summer and need nothing from you.
Step 2: Net the pond before the leaves fall
Falling leaves are the enemy of a healthy winter pond. When they sink and decompose, they foul the water, feed spring algae, and consume the oxygen your fish need under the ice. Stretch a net across the pond in early autumn, before the trees let go, so the leaves land on the mesh instead of the bottom. Then you simply lift the net and shake them off. The same netting deters herons, which still hunt ponds in the cold. If you already covered the pond for predator protection, you are ahead of the game.
Step 3: Clean out debris
Before the cold sets in, remove the organic load that would otherwise decay under ice. Less waste means less gas, less oxygen demand, and cleaner water come spring.
- Skim and net floating leaves and twigs off the surface.
- Remove sludge and muck from the bottom with a net or pond vacuum, especially the deep zone where waste collects.
- Cut back and remove dying plant foliage so it does not rot in the water.
- Trim hardy marginals and lower potted plants to the deep zone if they overwinter submerged.
- Rinse and clear the skimmer basket one last time before any shutdown.
A lighter waste load also means a lighter bioload going into the cold, which keeps the water more stable. If you want to understand why decaying matter is so risky under ice, our explainer on the pond nitrogen cycle connects the dots.
Step 4: Add a de-icer and aerator
The single most important winter task is keeping a hole open in the ice. A fully frozen surface seals in toxic gases from decomposing waste and shuts out oxygen, which can suffocate koi during a long freeze. A floating de-icer keeps a small patch ice-free so gases escape and air gets in. Pair it with an aerator bubbling near the surface to hold that opening more reliably and add oxygen. Never smash the ice to make a hole, because the shock can injure or kill the resting fish below. Size aeration to your pond with the pond aeration calculator.
| Tool | Job in winter | Placement |
|---|---|---|
| De-icer / floating heater | Keeps a hole open for gas exchange | Floating at the surface |
| Aerator / air pump | Adds oxygen, helps hold the hole open | Air stone near the surface, not the bottom |
| Netting | Keeps leaves and predators out | Stretched across the surface |
| Deep zone | Warm, stable refuge for dormant koi | 3 ft+ deepest part of the pond |
Step 5: Decide what to do with the pump
Your pump strategy depends on your climate. In mild winters where the pond never freezes hard, many keepers simply run the pump and filter year round. Where freezes are severe, a running waterfall can draw warmth out of the deep zone and build dangerous ice dams, so the common approach is to shut down the main pump, remove it, and store it in a bucket of water indoors so the seals do not dry out. Then rely on the de-icer and a separate aerator through the cold.
If you do keep aeration running, position the air stone near the surface rather than at the bottom. Bubbling cold air through the deepest water chills the very refuge where your koi are resting, which is the opposite of what you want.
Step 6: Protect the deep zone
The deep zone is your koi's winter survival shelter. Water at the bottom of a 3-foot-plus pond stays warmer and more stable than the surface, giving koi a calm place to rest in a near-dormant state until spring. In regions with hard, sustained freezes, 4 feet or more is safer. Keep the deep zone clear of sludge so the water there stays clean and oxygenated, and avoid disturbing the fish. If your pond lacks a real deep zone, overwintering koi outdoors is genuinely risky, and moving them indoors for the season may be the safer call.
Through the winter and into spring
Once the pond is winterized, your job is mostly to leave it alone and check on it. Make sure the de-icer keeps a hole open after every hard freeze, top off evaporation with dechlorinated water, and keep the net clear of leaves and snow load. Do not feed, do not break the ice, and do not stir up the deep zone. When spring returns and the water climbs back above 50 degrees and holds, restart the pump, clean the filter, and resume feeding slowly with small amounts. For deeper detail on cold-weather koi care, see our guide on overwintering koi, and plan your seasonal tasks with the pond aeration calculator so oxygen never runs short.
Pond Build & Maintenance Planner
Build planner, stocking planner, water-test log, and seasonal maintenance schedule, in one printable planner that keeps your pond healthy year-round.
Frequently Asked Questions
When should I stop feeding my koi for winter?
Stop feeding once the water temperature drops below about 50 degrees Fahrenheit and stays there. Koi are cold-blooded, so as the water cools their metabolism slows and they can no longer digest food properly. Feeding cold fish leaves undigested food rotting in their gut, which is dangerous. Use a pond thermometer rather than air temperature, switch to an easily digested wheat-germ food in the 50s, and stop entirely below 50.
Do I need to keep a hole open in the ice?
Yes. A sealed sheet of ice traps toxic gases from decomposing waste and blocks oxygen exchange, which can suffocate fish over a long freeze. A floating de-icer keeps a small area ice-free so gases escape and air gets in. Pairing it with an aerator bubbling near the surface keeps that hole open more reliably and adds oxygen. Never smash the ice, since the shock waves can harm or kill resting koi.
Should I run my pump and waterfall through winter?
It depends on your climate. In mild winters many keepers run the pump year round. Where it freezes hard, a running waterfall can pull warmth from the deep zone and create dangerous ice dams, so many people shut the main pump down, remove it, and rely on a de-icer and a separate aerator instead. If you do stop the pump, position aeration near the surface, not at the bottom, to avoid chilling the deep refuge where koi rest.
How deep does a pond need to be to overwinter koi?
Koi need a deep zone of at least 3 feet, and 4 feet or more in regions with hard, sustained freezes. The deep water stays warmer than the surface and gives koi a stable spot to rest near the bottom in a near-dormant state through the cold. A shallow pond can freeze too far down and leave no safe refuge. If your pond lacks a deep zone, overwintering koi outdoors is risky, and indoor housing may be safer.
Why put netting over a pond in fall?
Autumn leaves that sink and rot foul the water, feed algae, and consume oxygen under winter ice, so keeping them out matters. Stretching netting across the pond before the leaves drop catches them so you can remove them in bulk instead of fishing decayed debris off the bottom all winter. The same netting also deters herons and other predators. Remove and clean out any leaves that do collect before they break down.
Do koi need food at all over winter?
No. Below about 50 degrees Fahrenheit, healthy koi enter a near-dormant state and live off reserves built up during the warm months, so they do not need feeding through winter. Their digestion essentially shuts down, and offering food does more harm than good. Resume feeding only in spring once the water warms back above 50 and holds there, starting with small amounts of an easily digested food.
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