Most winter koi losses aren't from the cold. They're from October. Specifically, from the decisions that went unmade in October. The owner kept feeding high-protein food into water that had already dropped to 9 degrees, because the fish were still surfacing and it seemed wrong to stop. Or noticed something developing on a flank in mid-September, put off treating it, and by January the water was 4 degrees and the fish's immune system was at a fraction of its summer capacity. Or didn't get round to cleaning the filter before temperatures fell far enough to slow everything down. It's the same pattern. Every year. Ponds all over the UK.
When to Start Winterising Your Koi Pond
Water temperature, not the date. That's the signal.
In the UK the drop below 15 degrees Celsius happens between late September and early November, and the speed of it varies considerably. Some autumns it crawls down over six weeks. Others you get one cold fortnight and the pond is suddenly at 11 degrees before you've switched the food. Air temperature above the water isn't a reliable guide. The water holds heat longer than the air does, and a thermometer in the pond is the only way to know what's happening at fish level.
Below 10 degrees the fish slow dramatically. Digestion is largely stopped. The immune response is suppressed. A bacterial infection that a healthy koi in July would shrug off can take hold in weeks when the water is cold and the fish's defences are down. The preparation window closes faster than most owners expect.
Adjusting Koi Feeding in Autumn
High-protein food at 8 degrees doesn't get digested. It sits. That unprocessed food in the gut causes internal damage you won't see from the surface, and whatever portion the fish didn't eat sinks to the bottom and rots, loading the water with ammonia at the exact point when your filter bacteria are also working at reduced capacity. Both problems happen at the same time. And they make each other worse.
Switch to wheat germ food when the water hits 15 degrees. It's lower in protein and formulated for the cold-water gut that koi develop through autumn. Small amounts only. If the fish aren't actively surfacing to feed, don't put food in.
Below 8 degrees: stop. Full stop. The fish aren't hungry in any functional sense. They're not processing food. Feeding them at that temperature isn't kindness. Resume in spring only when temperatures are reliably back above 10 degrees and the fish are visibly active.
Should You Run the Pump and Filter Through Winter?
For most UK koi ponds, yes.
A running pump keeps water moving, reduces the chance of the surface sealing over completely, and keeps the filter bacteria alive. Those bacteria don't get through a full winter shutdown. A filter that's been off since November will be starting from nothing in March, at the exact moment when fish are waking up and producing waste again. A brand-new biological filter in a pond full of koi coming out of winter is a serious situation.
In exposed locations where frost is a real concern, insulate the exposed pipes and fittings properly. That's a better answer than switching the pump off. If you do turn it off for any extended period, run it for twenty minutes once a week. The bacteria in the filter media need water flow to survive, and a dormant filter is much harder to rescue than people realise.
Preventing Ice and Maintaining Oxygen
Fully frozen means sealed. Ammonia and methane from decomposing waste on the pond floor have nowhere to go. Oxygen can't get in. Given long enough under a completely sealed surface, koi asphyxiate at temperatures that aren't even close to lethal. The temperature isn't the danger. The ice is.
A de-icer keeps one section of water open. That's the whole job. Not heat the entire pond. Not keep half the surface clear. Just a hole.
Don't hammer the ice to break it. A solid blow sends a shockwave through the water that can injure fish two or three feet below where you're standing. A kettle of hot water poured over one spot melts a gap without the shockwave. Use a de-icer from the start of cold weather and you won't be doing this in your garden at 6am in January.
Run an air stone at shallow to mid depth in winter, not at the very bottom. Pulling cold water up from the deepest part of the pond disrupts the warmer layer where the fish shelter through cold spells. They need that layer. Don't disturb it.
Should I cover my koi pond in winter?
In an exposed position, or anywhere in the north of England or Scotland, yes. A pond tent or solid cover holds heat, significantly reduces the chance of a full surface freeze, and keeps leaf fall out of the water through autumn and winter. Leaves sitting on the pond floor for months decompose and load the water with ammonia. Any cover needs to allow gas exchange. A sealed airtight cover creates a different version of the same oxygen problem it was supposed to prevent.
Caring for Your Pond Viewing Window in Winter
Quality pond glass doesn't need winterising. The one practical job is cleaning the external face in October, while being outside next to a pond for five minutes is still something you'll actually do willingly.
Algae and summer debris comes off far more easily at 12 degrees than at 4. Leave the glass until February and you've made the job harder and had a murky view all winter for no reason. Clean it going in. Our article on how to keep your pond viewing window crystal clear covers the full maintenance routine.
Treating Health Issues Before the Cold Arrives
September. Look at every fish. Properly.
A koi's immune system starts suppressing as water temperature falls through autumn. An infection that was sitting quiet at 18 degrees in September becomes a genuinely serious problem at 6 degrees in January, in a fish whose body has very limited capacity to respond to anything. Treatments that work at 15 degrees have reduced efficacy at 10 degrees and often stop functioning properly below 8. There's a window. It's September and early October. A fish going into winter with an unresolved ulcer or parasite burden is at substantially higher risk of not making it through. Some don't.
Winter Koi Pond Checklist
Before temperatures drop below 10 degrees
- Switch to wheat germ food from around 15°C
- Test water quality fully: ammonia, nitrite, nitrate, pH, and KH
- Clean pond filters and check pump is operating correctly
- Clear fallen leaves from the pond surface and check the pond net
- Lag exposed pipes, valves, and equipment against frost
- Install or test a pond de-icer or floating heater
- Place an air stone at shallow to mid depth connected to a running air pump
- Clean your pond viewing window and inspect the seal
- Check jump guards are securely in place
- Treat any unwell fish while water is still warm enough for treatment to work
- Stop feeding entirely below 8°C
What temperature is too cold for koi in the UK?
Koi survive just above freezing, but the risk becomes serious once temperatures sit at or below 4 degrees for any meaningful stretch. Temperature alone isn't the whole story. It's cold water combined with a suppressed immune system and declining water quality that kills fish. Koi that go into winter in good health, in a clean and well-filtered pond of adequate depth, come through it. The ones that don't almost always had problems going in that October didn't catch. For the depth question, our guide on how deep a koi pond should be covers it properly.
Making Winter Work for Your Pond
The preparation isn't complicated. Change the food at 15 degrees. Stop feeding at 8. Keep the water moving and the surface from sealing over. Look hard at the fish before October is out and deal with anything you find while the water is still warm enough for treatment to work.
What goes wrong is the assumption that the pond will manage itself through November and December the way it managed itself through June. It won't. Winters in northern England push pond temperatures into genuinely dangerous territory for weeks. Even in the south, three or four consecutive nights of hard frost is enough to seal a shallow or poorly prepared pond into a real problem.
The difference between koi that make it through and koi that don't is almost always a decision made eight weeks before anything visible went wrong.
For pond accessories to help manage your pond through every season, browse our pond accessories collection, jump guards, and glass viewing windows. For more practical koi keeping guides, visit the North Glass blog.
