Winter’s Coming… Are Your Koi Ready?

Author: Joe @ The Koi Perspective

5 Essential Tips for Winter Koi Pond Care

Winter is one of those times of year where it feels like nothing is happening in the pond - but actually, it’s the season that tests how well you’ve set everything up. And let’s be honest: this is the point in the year where most of us would love to forget we even own a pond. It’s freezing, it’s dark by 4pm, and who on earth genuinely enjoys standing outside in sideways rain trying to do a pH test while your hands stop working?

It’s very easy to fall into the “I’ll check it tomorrow” routine… and then tomorrow becomes March.

Meanwhile, in Japan, even at the big farms, winter isn’t a time to switch off. Breeders still check their ponds every single day, no matter the weather. I asked one breeder how often he checks in winter, expecting him to say “every other day at most” … nope. Still daily. No excuses. He just shrugged and said something along the lines of: “The koi don’t take a day off, so neither do we.”

And unfortunately, he’s right. Small issues in cold water can become big issues by spring; and they creep up silently because nothing looks urgent. The fish go quiet, the water looks the same from one week to the next, and suddenly you realise you haven’t actually looked properly at the koi since Christmas.

So yes, winter is the perfect time for the hobbyist to get lazy. But it’s also the time the pond needs you the most - even if it means putting your big coat on, pretending the rain isn’t going down your neck, and doing that quick check before dashing back inside.

Here are five simple but important things to stay on top of:

Daily Koi Health Checks: Spotting Issues in Cold Water

You don’t need to be out there every hour, but a quick look each day goes a long way. Even in cold water, you can still spot early warning signs such as a koi isolating on its own, one that’s hanging directly under a return, any marks or lifted scales that weren’t there last week, or a fish that just isn’t behaving like it normally does.

If your water is sitting around 8-12°C, the fish shouldn’t be completely switched off. They might be slower, but koi are koi - they’re programmed to look for food. They graze constantly, picking at algae, weeds, biofilm… anything that looks remotely edible. Koi are ‘continuous foragers’..., and that natural behaviour doesn’t vanish just because it’s cold.

So, if you drop a tiny handful of wheatgerm food, or a highly digestible diet (for cold temperatures) in and the whole pond comes up calmly and confidently, that’s a good sign. 

Continuous Foraging: Using Food to 'Read' Your Koi in Winter

You’re not feeding to grow them; you’re feeding to read them. A koi that still shows interest in food is usually a healthy koi. A koi that ignores food completely when the temperature isn’t that low? That’s when you need to start paying attention.

Japanese breeders rely heavily on this. They don’t need lab equipment or fancy gadgets to know if something’s off, they just watch how the fish respond to food. Desire to feed tells you a lot, even in winter.

If you leave the pond alone all winter and only check it when something smells wrong… you’re already in trouble.

Koi Winter Feeding Strategy: When and What to Feed Below 10°C / 50°F

Once your pond drops below 10°C / 50 ℉, feeding becomes more about judgement than routine. Some keepers stop completely, letting the fish rely solely on natural food in the pond. Others offer small amounts of wheatgerm or other low-protein foods if the koi are still showing interest.

The Golden Rule of Cold-Water Feeding: Avoid Leftovers

There’s no one-size-fits-all rule - but here’s the golden rule: don’t leave leftovers in the pond. At low temperatures, uneaten food rots slowly and can do far more damage than skipping a meal. Cold water slows down digestion, so even a little excess can quickly foul the water.

Choosing Low-Protein, Highly Digestible Winter Diets

Winter is also the time to think carefully about what type of food you’re giving. High-protein diets designed for growth that you might feed in spring or summer are completely unsuitable in cold months - they can stress the koi and cause digestive issues. Low-protein easy to digest foods are ideal because they provide slow energy and keep the fish ticking over without overloading their systems.

But if you want to be more precise, especially with premium koi or smaller stocks, it’s worth speaking to your Japanese koi specialist. They can advise on the exact brand, formulation, and quantity that’s best for your pond, factoring in your water temperature, natural foods, and the koi’s condition. A bit of expert guidance here can save you a lot of headaches and help the fish stay healthy all winter.

Feeding in winter isn’t about growth, and to be honest the only thing that’s likely to happen is you will cause your fish to get fat if you over feed in cold temperatures - it’s about keeping your koi healthy, active, and ready to hit the spring in top shape, with a fighting fit immune system.

Winter Pond Filtration & Aeration: Why You Can't Turn Pumps Off

A big mistake people make is switching pumps off to “save electricity.” I get it - the temptation is real. It’s cold, the bills are creeping up, and the fish don’t seem to be doing much. But all that does is let waste and nasties build up quietly under the surface. Even if you’re not feeding, koi are still producing ammonia. Yes that invisible stuff that makes water smell and stresses fish, is still happening. Fish still breathe, excrete, and drop tiny bits of waste, even when they’re half-asleep at the bottom of the pond.

Managing Ammonia Build-up with Slowed Filtration

A filter that’s ticking over slowly is far better than one that’s off for three months, and will help when you come to ramp up the feeding in the springtime. The water doesn’t need to be roaring through a massive pump. Now, I’m not against turning the pumps down & that is a major benefit of the Oase systems is the ability to turn down the pumps to save on electricity - just enough to keep it moving and passing through media, so ammonia doesn’t accumulate. Cold water actually slows down the breakdown of waste, which means a shutdown filter equals a very slow, very quiet disaster waiting to happen.

Essential Aeration to Prevent Dead Spots and Ice

Aeration is just as important. Cold water holds more oxygen, but koi still need movement to circulate the pond, prevent dead spots, and stop the water from stratifying. Even a small air pump bubbling away in a corner can make a huge difference, it can also stop the water from freezing on the surface of the pond.

Japanese farms run aeration non-stop throughout winter. They don’t do it because they’re obsessives (well… maybe a little), they do it because clean, moving water equals healthy koi - even if the fish aren’t doing much else. Essentially, your filter and aeration are the silent winter bodyguards for your pond. Ignore them, and you’ll notice the consequences come spring… and by then, it’s a lot harder to fix.

Pre-Winter and Spring Health Checks: Parasites, Redness, and Mucus Biopsies

Before winter lands, make sure your fish are going into the cold months in good condition. It’s far easier to fix minor issues in autumn than to try and sort them out in January when the fish are sluggish and the water’s freezing. Take a proper look at each koi for these attributes:

• Redness?
• Thinning?
• Odd behaviour?
• Isolation?
• Raised Scales?
• Heavy Mucas coating?

If any one of these things is occurring on even one koi, it could be the start of major issues down the line, especially if ignored or missed, start by taking a water parameter check, if these parameters seem ok, then the next step is to take a mucas biopsy from the sick koi and check it under a microscope for any sign of skin or gill parasites.

Once you hit spring and temperatures start creeping up, repeat the same check. This is when problems suddenly show themselves - parasites wake up quicker than koi do and can take advantage of the koi’s weaker immune system having been through a tough winter period.

Japanese breeders expect spring issues, not because their systems are bad, but because that temperature swing is always a risk, and some koi handle winter better than others. It's like humans, we all get the winter bug at some stage (or at least I do!) 

This is where a pond window can be invaluable. Even if your pond is covered for insulation, being able to look directly through the water without disturbing the fish with a net makes spotting small problems much easier. Some people put covers or nets on their ponds, and while they do help with winter protection, they can make it tricky to notice subtle things like a little redness under the mouth or a blister on the belly. These small signs are easy to miss until it’s too late, so take your time and get as close a look as possible when the weather allows.

Once spring arrives and temperatures start creeping up, repeat the same checks. This is when issues can suddenly show themselves, because parasites and other winter-hibernating nasties wake up faster than your koi do.

Think of it as a gentle wake-up call for your pond: a careful, hands-on look now saves a lot of stress later.

The key is spotting things early.