Short answer: winterizing a koi pond comes down to four things. Clean out the fall gunk before it sinks, stop feeding once the water drops below 50°F, keep a hole open in the ice so gases can escape, and let your koi rest until spring. That's it. No panic, no heavy machinery.
The details depend on where you live. A pond in Minnesota and a pond in Florida need very different winters, so we've split the checklist by climate below.
Why does a koi pond need winter prep at all?
When the water gets cold, everything in your pond slows down. Your koi eat less and move less. The good microbes that clean your water slow down too. Meanwhile, the leaves and fish waste that pile up in fall keep breaking down on the bottom.
Under a layer of ice, the gases from that breakdown have nowhere to go. A little prep in fall means less muck going into winter, and a healthier pond waking up in spring.
Which winter does your pond have?
| Your winter | Typical places | What it means for your pond |
|---|---|---|
| Cold: the pond freezes over | Upper Midwest, Northeast, Mountain West | Full winterizing: de-icer or aerator, waterfall off, no feeding for months |
| Mild: occasional ice | Mid-Atlantic, Upper South, Pacific Northwest | Light prep: keep an eye on cold snaps, feed lightly on warm days |
| Warm: water rarely drops below 50°F | Florida, Gulf Coast, Southern California, Arizona | Koi stay active: keep feeding lightly and keep the filter running |
Step 1: Clean up before the cold sets in
Fall is when ponds collect the most junk. Do this while the water is still above 50°F:
- Net the leaves. A pond net stretched over the water saves you hours of scooping.
- Trim back plants. Cut dead and yellow leaves from water lilies and marginal plants so they don't rot in the water.
- Remove the muck. Scoop or vacuum the worst of the sludge off the bottom. You don't need a spotless pond, just less food for winter decay.
- Skip the big water change. A small top-off is fine. A full drain-and-refill right before winter stresses your fish.
A little help from us: Sanadrop POND helps your pond's own cleanup crew break down organic waste, so less sludge builds up through fall.
Step 2: Change how you feed
Koi digest food slower as the water cools, and uneaten food just turns into more muck. Use a pond thermometer and follow the water, not the calendar:
| Water temperature | What to feed |
|---|---|
| Above 60°F | Regular food, as usual |
| 50–60°F | Easy-to-digest cold-water food (wheat germ), smaller portions, every few days |
| Below 50°F | Stop feeding. Your koi will be fine until spring. |
It feels strange to stop feeding your fish, but in cold water they live off their reserves. Feeding them anyway does more harm than good.
Step 3: Keep a hole in the ice
If your pond freezes, the most important job of winter is keeping a small opening in the ice so gases can escape and oxygen can get in.
- Use a floating de-icer. It doesn't heat the pond. It just keeps one spot open.
- Add an aerator near the surface, not on the bottom. The deepest water stays a little warmer, and that's where your koi like to rest.
- Never smash the ice. The shock wave can hurt or even kill your fish. If the hole closes, melt it gently with a pot of hot water set on the ice.
Step 4: Decide what to do with pumps and waterfalls
In cold climates, most pond owners shut down waterfalls and streams for the winter. Ice can build up and push water over the edge, draining your pond overnight. If you keep a pump running, lift it off the bottom so it doesn't stir up the warmer deep water.
External filters, UV clarifiers and plumbing lines that can freeze should be drained and stored. In mild and warm climates, keep the filter running all year.
Step 5: Let your koi rest
Koi spend winter near the bottom, barely moving. That's normal. Your pond should be at least 18 to 24 inches deep so they have a stable spot to rest. Watch them from a distance and resist the urge to check on them up close. A calm winter is a healthy winter.
Your winter checklist
- Net leaves and trim plants
- Remove the worst of the sludge
- Switch to cold-water food at 60°F, stop at 50°F
- Install a de-icer or aerator before the first freeze
- Shut down waterfalls and drain external equipment (cold climates)
- Make sure the pond is at least 18 to 24 inches deep
- Leave your koi in peace until spring
Winter pond FAQ
Do koi need a heater in winter?
Usually not. Koi are cold-hardy and do well in winter as long as the pond is deep enough and a hole stays open in the ice. A de-icer is not a heater: it only keeps the surface open.
Should I feed my koi in winter?
Not once the water is below 50°F. Their digestion slows down so much that food sits uneaten and pollutes the water.
Is it OK if my pond freezes over?
A frozen surface is fine as long as you keep one opening for gases to escape. A pond that freezes over completely for days is the real risk.
Can I turn off my pump in winter?
In cold climates, yes, as long as a de-icer or aerator keeps a hole open. In mild and warm climates, keep the water moving.
When do I start up my pond again?
When the water is back above 50°F for several days. We'll cover that in our spring start-up guide.