Winter is a time when everything in a pond slows right down, including your fish! After an entire summer of feeding and thriving, fish start to prepare for hibernation (technically, a state called “torpor“) as colder weather draws close.
If you have a pond with fish, especially koi, you should consider steps to help “winterize” your pond, which will create a more comfortable environment for them during winter. This goes two-fold if you live somewhere with particularly harsh winters, as not preparing your pond correctly can lead to very sick fish and poor water conditions come spring.
There are a range of things you can do to prepare for winter which will give your koi a much happier and healthier time while in torpor. Some things may only apply to areas with very cold winters, but all can be incorporated in most ponds to improve the pond’s winter eco-system.
What Do Fish Do In the Winter?
In warmer temperatures fish have much higher metabolisms, which is why they’re very eager to eat during the summer months. As temperatures begin to drop during the autumn, koi will start eating less, and feeding should be reduced as this happens. When the pond water reaches around 50ºF (10°c), the metabolisms and digestive systems of the fish will all but shut down for the winter – they will essentially start hibernating (enter torpor).
While in torpor, most of the koi’s bodily processes will slow down, allowing them to survive on the minimal of food and energy requirements. You’ll usually see them swimming slowly around the deepest point of the pond, which is also the warmest part during winter. Even though they’re effectively “shut down” from actively during this time, they will still need to move on occasion to prevent their joints from seizing up, and also may come to the surface if there is an ice-opening for extra oxygen. If you notice your koi floating in water motionless for a long period, this is also normal, so you shouldn’t panic! Fish in winter are running on such little energy requirements, you’ll often only see them moving when absolutely necessary.
During this state, koi cannot digest food effectively, and will not actively search for it until warmer weather arrives. Even if you see them swimming around the pond, you should not feel the urge to feed as they won’t likely take it and it’ll just contribute to waste on the pond floor. So long as they have been well fed in summer they should have plenty of fat stores to see them through!
Is A Frozen Pond Dangerous to Koi?
- De-Icer for ponds whose temperatures are down to 20 degrees Fahrenheit
- Highly-visible LED pilot light that indicates when unit is on
- Integrated thermostat shuts off the De-Icer when it overheats, protecting internal components from damage
A pond freezing over is natural, and so long as you’re prepared there should be no issues with your koi. Pond ice can actually be a benefit, as it acts as nature’s pond cover, helping to keep water temperature stable. One of the biggest hurdles of koi fish keeping is maintaining a stable eco-system, as koi are so sensitive to rapid changes in the environment. A frozen pond actually helps in this regard by protecting the pond from the much colder air, and helping keep the water temperature within a narrower range over the winter period.
With that said, there are some immediate problems with a pond frozen solid which need to be considered if you’re keeping koi. The first being that if the pond is 100% frozen, no gas exchange can take place on the water surface. This means that oxygen cannot enter the pond, and harmful gases, such as carbon dioxide, cannot leave. This won’t be a problem initially, but a slow build up in CO2, and a decrease in O2, can cause serious problems for fish in their torpor state (similar to hibernation) over winter.
As well as fish, beneficial bacteria that work over the winter require oxygen to break down organic waste, and without sufficient oxygen, you may have a large build-up of harmful (and smelly) substances come spring. In the absence of oxygen, normal aerobic bacteria cannot break down substances and slow-digesting anaerobic bacteria take over. The problem with this process is the slow-digesting bacteria build large amounts of by-products, such as carbon dioxide and hydrogen sulfide, which can make fish sick and cause a rotten egg smell when water mixes in spring.
To prevent these issues, gas exchange via an opening in the ice and some form of aeration is usually recommended for koi ponds. Both are explained in more detail below!
Koi Winter Feeding Guidelines
As a rule of thumb, you should not feed koi during winter at all. During the build-up to winter, their feed should be gradually decreased as the temperature begins to drop. The colder it gets, the slower the koi’s digestive system becomes, which means they can digest less and less food. When the water temperature reaches around 50ºF (10°c), you should stop feeding koi as their metabolism and digestive system will be all but shut down.
Over winter they’ll survive in a low energy state and slowly consume their fat stores which they’ve built up throughout the year. Due to this, one of the most important things to prepare for winter is to make sure your koi are eating a high-quality summer feed so they have plenty of fat stores for a comfortable winter! This is far more important than what to feed them in autumn, as they will naturally be eating much less of what you give them, anyway.
The rate at which koi can digest food is directly related to the water temperature, so you can start slowly decreasing the amount of food as water temperature drops throughout autumn. As a general rule we advise to stop feeding koi altogether once the temperature of the pond water drops to 55ºF (13°c) and remains there for more than 7 days. Even if the temperature later rises above this, you should not start feeding again until spring arrives.
In summary:
- Reduce down feeding through autumn, cutting to only 2-3 feeds a week
- When water temperature reaches 55ºF (13°c) and remains for more than a week, stop feeding completely
Should I use wheatgerm feed in winter?
- Wheat germ transitions fish in and out of winter
- Exceptional digestability
- For colder months
Wheatgerm can be easier to digest for koi in comparison to regular feeds, as well as being packed with healthy fats and slower release carbohydrates. This makes it ideal for feeding during the buildup to winter; but is it worth replacing your usual koi feed altogether?
If you want to try wheatgerm and see how your koi like it, we say do it! If not, just continue to feed with regular food and gradually reduce the amount as it gets colder. Koi won’t eat just for the sake of eating, so keep an eye on the behavior of the fish and feed as appropriate.
Should I turn off my Pump and Filter in Winter?
Turning off pond equipment will depend on how cold and harsh your climate is where you live. Typically in places such as the UK, winters do not get cold enough to damage equipment, and many pond owners will choose to run their pump and filter all winter for extra aeration. Problems occur when the temperatures are so cold that the water freezes solid inside the piping and main equipment chambers.
As water freezes, it expands slightly, and this can cause cracks and damage to pipes, filter boxes, and especially pumps. For northern parts of the US and Canada, for example, temperatures can drop far below freezing, so turning off equipment and storing it over winter is usually recommended.
What about waterfalls and fountains?
Waterfalls and fountains may be able to keep your pond free of ice and provide extra aeration, but they also act as large heat exchangers, greatly cooling pond water. One of the most important aspects of koi keeping is creating a stable eco-system, which includes a stable water temperature.
Not only will waterfalls and fountains cause heat loss at the surface of the water, they will also stir up the warmer water in the deeper areas of the pond, decreasing the temperature. This is the area koi will spend much of their time over winter, and it needs to remain stable and warmer than the surface water. Even though water features do look impressive in winter and can appear beneficial, it’s recommended to switch them off for a more stable water temperature.
Does This Guide Apply for Goldfish, too?
Common Goldfish are hardier than koi and can withstand harsher changes that koi may suffer under. For example, goldfish can survive in a wider pH range in comparison to koi, which is one of the many dangers of not winterizing a pond correctly. They can also survive at lower temperatures and can adapt more quickly to changes in water quality.
Also, since goldfish are smaller than koi, many pond owners will even choose to bring their fish indoors into a holding tank over winter. This, as you can imagine, is not always an option for koi owners due to the size of the fish and the amount of waste buildup that would occur.
With that said, goldfish still have their limits, and all information in this article that applies to koi can also be used to help winterize a goldfish pond if the fish are kept outdoors. Winterizing a goldfish pond will give your goldfish a much more comfortable winter period, just the same as a koi pond. To read more about some of the differences between goldfish and koi, check out our in-depth guide here.
How to Winterize a Pond for Koi & Goldfish
1: Big Autumn Pond Clean
When? Throughout the entire of autumn.
- Max; Suction Depth: 7 Ft
- Suction Hose Length: 16 Ft
- Discharge Hose Length: 8 Ft, Max; Flow Rate: 1300 Gph
One of the most important steps to prepare for winter is to start with a big pond clean throughout autumn. Leaving large amounts of organic waste in a pond will be left to fester all winter under the ice, and will gradually build harmful substances. Koi are extremely sensitive to swings in water quality, with a change in pH and a rise in ammonia, both from waste, being the most dangerous.
Since beneficial bacteria will slow down during the winter and your filter system may be switched off, it is very important to give the pond a good clean before the cold weather hits.
This deep clean should begin at the start of autumn when temperatures begin to drop, and involves removing bottom muck, algae, and trimming back pond plants. Bottom muck, or pond sludge, is the buildup of all organic matter which sinks to the bottom of your pond and starts to decompose – this includes leaves, fish waste, and any excess fish food.
Beneficial bacteria will constantly work to break this waste down, but they won’t likely be able to remove all of the sludge by themselves. At the start of autumn we recommend adding a sludge remover treatment to the water and then vacuuming or raking out the excess. You can also just skip the sludge remover product and just use your vacuum or rake, but we find supplementing with more bacteria to help break down sludge helps during winter.
As well as sludge, algae should be maximally treated as it can still grow through autumn. A UV clarifier running for a few weeks should be perfect for the job. Just make sure to also regularly clean your filter media as the algae die off as it will quickly become clogged.
A pond net should also be installed to catch falling leaves and debris in autumn, which will make cleaning the pond much easier. The net should be removed before it gets too cold, however, or it will freeze in place. Netting is only used to aid in the cleaning process and prevent a buildup of new waste due to falling leaves in autumn.
2: Threat for Parasites or Infections
When? 2 months before water temps drop to 50ºF (10°c).
- Contains one (1) API MELAFIX Freshwater Fish Bacterial Infection Remedy 64-Ounce Bottle
- Heals bacterial infections and repairs damaged fins, ulcers and open wounds
- Contains natural, botanical tea tree extract to quickly and rapidly help fish
Contrary to common belief, fish are at a far higher risk of parasitical and bacterial infections during winter. During hibernation/torpor where koi lower their digestive system, they also have a greatly reduced and more vulnerable immune system. If koi have an infection going into winter where their immune system starts to shut down, you will end up with a very sick fish! Due to this, it is best to treat any possible signs of parasites or infection before the water temperature can drop too low.
We recommenced treating your fish 2 months before they begin torpor so they have time to fight any current bacteria or parasites before winter. Doing this will also provide longer immunity over winter, as treatments usually have an extended period of immunity afterwards.
Koi “flukes” are a very common parasite which infests the fish’s gills, with symptoms being rapid swimming and rubbing against rocks and pond liner. Flukes are microscopic, so you can’t see them physically with the naked eye. During winter their eggs can lay dormant for months and cause major issues in spring, so it’s very important to threat koi for flukes in both fall and spring for maximum immunity. We recommend Aqua Meds Prazi Fluke treatment for koi ponds.
Bacterial infections can be treated with APIs MELAFIX for ponds, and we recommend running this alongside fluke treatment. This treatment covers a range of bacterial infections and also promotes healing with natural active ingredients.
3: Install A Pond De-icer (Heater)
When? Before the pond water freezes over.
- De-Icer for ponds whose temperatures are down to 20 degrees Fahrenheit
- Highly-visible LED pilot light that indicates when unit is on
- Integrated thermostat shuts off the De-Icer when it overheats, protecting internal components from damage
Keeping a breathing hole open in the pond ice is one of the most important steps to safely winterizing a pond. A hole allows for gas exchange to take place, allowing harmful gases such as Co2 to escape and oxygen to dissolve from the air. Even if you’ve carried out a heavy pond clean, there will still likely be a small amount of waste present.
Koi will also continue to produce waste over winter, although at a greatly reduced rate. Without a sufficient means of gas escape, even a small amount of ammonia build up can seriously stress your koi, so it’s important we keep a hole open all winter to reduce this risk.
The easiest way to do this is to install a floating pond de-icer before the pond water freezes over. The heater will float on the surface water and slightly raise the temperature of its surroundings, allowing a constant hole in the ice to remain open. Koi will also appreciate the slightly warmer temperature, and you’ll often see them hugging the heater and coming up for air occasionally.
De-icers are not designed to heat the entire pond water, but if a powerful de-icer is installed in a small pond it may literally cook your fish! Never place a de-icer in a small pre-formed pond, and always make sure the unit you buy works with your current pond size. For a list of our recommended deicer models, check our full guide on this here.
4: Consider Adding An Air Pump (Oxygenator)
When? After your autumn pond clean up.
- Aeration for 2,000 - 8,000-gallon ponds and water gardens
- Max operating depth is 4 feet deep
- Provides 0.80 CFM airflow with 1 Dual Stick Plate
Even with a de-icer, oxygen content in winter will be naturally lower, especially if 90% of the pond is covered in ice. Both koi and beneficial bacteria still need oxygen to function, so the more stable the supply, the better! Adding an air pump to your pond over winter is a great additional step which provides extra oxygen, aeration, and movement which helps prevent ice forming.
If you have a large number of koi, we highly recommend running an air pump alongside a de-icer. Having both will create a much more comfortable environment for koi, providing plenty of oxygen throughout their winter period.
Unlike a de-icer which is unlikely to ever freeze (unless it breaks down), an air pump should be sheltered from the cold if you decide to run it all winter. Positioning the air stones close to the de-icer will make sure they continue to work, and keeping the main pump under a cover will prevent snow and ice damaging the unit.
Whereas a waterfall or fountain causes great surface heat loss, an air pump can aerate the water without any significant temperature problems, so is ideal for helping maintain a stable winter eco-system. For more information on air pumps and our recommended picks, check our full article here.
5: Supplement with Winter Bacteria
When? Throughout Autumn in preparation for Winter.
- GET READY FOR COOLER WEATHER: Microbe-Lift Autumn/Winter Prep is a specifically formulated beneficial treatment to winterize your pond to...
- WATER CLARIFIER: Autumn/Winter Prep accelerates the breakdown of leaves, sediment and other organic matter during the fall and winter...
- HOW IT WORKS: Autumn/Winter Prep is a two-part system of liquid bacteria and dry, water soluble packets containing a blend of enzymes and...
Beneficial bacteria will not only help with your autumn clean, it will also help keep waste levels to a minimum throughout Winter. Bacteria slow down during winter but don’t 
The best way to prepare your pond is to gradually supplement throughout autumn, which will allow beneficial bacteria populations to grow inside your filter and pond floor. Any beneficial bacteria product can be used for this purpose, for but convenience we recommend some kits which are designed to help in dosing for winter.
Microbe Lifts Autumn to Winter Prep kit contains a range of beneficial bacteria and enzymes which will help get your pond ready for winter and also aid in cleaning and maintenance. These kinds of treatments also help jump-start your beneficial bacteria and eco-systems when spring arrives, so are always a good addition to a pond.
Excellent Article!! I was searching information about koi winter care on Google. Then I reached on your blog, and there I found all info. It’s helpful blog for me. Thanks for sharing this kind of information.
I have recently acquired a pond with Koi and goldfish
I was grateful for the advice about feeding and winter care and will find it useful for this my first winter with fish
Thank you
Great advice for a new koi keeper. Thank you
Is it recommended to continue using any chemicals during winter months that are normally used doing warm weather months?
Hi Pat,
Thanks for reading and commenting! Most pond chemicals do best when water temperatures are above 60 degrees Fahrenheit. This is partially because water becomes more dense as it approaches the freezing point, making it more difficult for chemicals to mix and disperse properly. They’re also more likely to deplete dissolved oxygen levels in the winter. However, always read the labels of any chemicals that you have to be sure! They should legally have exact parameter specifications, including water temperature and pH levels, for safe use. For an example, feel free to take a look at this pond algaecide label: https://www.bestnest.com/bestnest/img_p/GOR-7361073.pdf
Excellent information! I really learned a lot and found that I should continue reading more about ponds, chemicals, koi life, etc – I was going to start a pond build this summer (2023) (I recently purchased a large preformed pond) but I think more education is needed. Thank you so much!
Thank for this post. It usful for me. Thank admin so much