How to Care for Ponds in Winter


During the winter months, the temperatures can become increasingly cold, but this does not mean you cannot have happy, healthy fish and a thriving pond. As long as certain measures are taken to ensure your pond is prepped for the change in season and cared for to keep everyone safe. 

When the temperature starts to noticeably drop, it's always a good idea to put a pond thermometer in to keep a closer eye on things. Once a pond's temperature starts to drop to 10 degrees Celsius it is time to take measures for winter. 

Read on to discover how to care for your pond over the winter months…

How to care for ponds in winter

Ensuring your pond remains in a healthy state can be a daunting task, but we’re here to help. 

See below for the answers to the most frequently asked questions when it comes to ensuring your pond stays healthy and habitable this winter. 

Do I need to clean my pond filter in winter?

Because of the change to diet and temperature through the winter months, the filter should not be working quite as hard as it does in the warmer months, as not as much waste should be passing through compared to the summer. The frequency for filter cleans, therefore, lessens. 

Depending on the size of your pond and stocking levels, you may only need to clean your filter once every month / three months. If your pond is struggling with the waste being produced in the pond over winter, the filter changes can be increased. This can be determined through water tests. We would recommend using the NT Labs Pond Test Kit for easy-to-use, accurate testing.

When cleaning any filter sponges, it is important to rinse in water taken from the pond NOT fresh tap water, as this can be detrimental to your pond’s health. 

Over the winter months, it’s not uncommon to see a small increase in the levels of nitrite in the pond as the cold temperatures can impact the good bacteria that have developed in the filter system. 

To combat this, when the temperatures start dropping, some slow-release bacteria pearls can be added to the filter system to help build back up the good bacteria throughout the colder months, and aid in a more efficient filtration system. 

Should I change my pond water in winter?

Whilst some people believe a complete pond overhaul is warranted in preparation for winter, this is wholly unnecessary. A small 10-20% water change when doing a filter clean is all that is required when ‘water changing’ the pond over winter.

Changing too much water in one go can be extremely stressful for the fish, and it can also alter the parameters too much in a short amount of time, which they cannot cope well with.

When adding any fresh water back into the pond, you should always use some dechlorinator to ensure that the water will be safe for the fish. Water quality is just as important in winter as it is in the summer, so regular cleaning and testing is essential.

Can I shut down my pond in winter?

Shutting down a pond over winter is not advisable. The pond still requires filtration and oxygenation even when it's cold and the fish aren’t as active. 

Steps can be taken to reduce the amount of equipment used during the winter months, for example, if you have multiple air pumps running, then these can be lessened. 

If you have a varipump then the flow control can be adjusted for a lower output. However, if all equipment was suddenly shut off there would be a toxic build-up within the pond which would not be able to be broken down naturally. 

Before changing anything too much, it is always advisable to speak to a professional for advice as every pond is different.

Can I turn my pond pump off in winter?

A pond pump has many purposes. Its main function is to remove the debris from the pond and send it to the filter system. This constant flow of water is beneficial year-round, but in winter provides additional advantages. The pond requires surface disruption and movement to prevent freezing over and oxygenation. 

A pond should never be left to stagnate. The constant movement of running water prevents this, and on very cold days, keeps a pocket of water ice-free to allow toxic gases to exchange at the surface of the pond. 

Most pond pumps can sustain temperatures as low as -20 degrees Celsius, so you should be able to leave it running over winter without issues. Only if the pond is very shallow, would it be advisable to remove the pump. However, this only tends to occur in water features and not ponds that house fish or wildlife. 

Can I leave my pond running over winter?

Over winter, the pond should be left running as normal to prevent any sudden changes to the water parameters. If you were to turn off the equipment for extended periods, it can cause severe ammonia and nitrite build-up, which takes a while to get back to safe levels and can be harmful to the fish.

Where possible, all equipment should remain on. If cutbacks need to be made, the filter and pump should be prioritised as they are the most important part of any pond set-up. Followed by additional sources of oxygenation, such as air pumps or fountains. 

If you’re looking to reduce running costs over Winter, we recommend installing a variable pump on your pond. The flow rate on variable pumps can be adjusted incrementally from 100% to 1%. 

As you reduce the power of the pump the energy consumption also decreases which will save you money on running costs. Rather than turning off your system which poses a threat to your fish, these pumps can be used at much slower rates during the winter keeping everything ticking over. 

As each pond is different, it’s important to seek advice on what would be best for you. Feel free to give us a call on 01204 582105 for all enquiries!

How do I stop my pond from freezing over?

It’s no secret that in the UK, our temperatures can really plummet. But don’t worry, there are many methods to stop your pond from freezing over. This includes:

  • Keeping your filter and pump running over the surface of the water creates disturbance and prevents the water from being too still, which helps to stop any ice from forming over the pond. 
  • Use a water fountain/air stones to maintain the concentration of oxygen in the water and create a pocket of movement on the pond surface to allow for oxygenation from the air above.
  • Floating ornaments placed around the pond will create constant movement, using the wind on the surface. This simple (yet effective!) solution will stop the ice from taking hold completely. 
  • Pond depth is key to ensuring temperatures remain safe. Ideally, your pond should be 18” or deeper. A deeper pond stays warmer for longer, whilst a shallow pond will freeze over pretty quickly.
  • Covering the pond is a simple yet effective method for trapping in the heat. It keeps the warm air, from the sunny days, trapped below the surface of the pond and retains the heat. It can also prevent leaves and other debris from falling into the pond and causing problems. Covers can be bought ready-made or built from various insulating materials.
  • Add a heater to the pond for the most consistent temperature year-round. It is the most expensive method but by far the most reliable. Most heaters have thermostats so it will remain at a set temperature until manually changed. 

You NEVER want to let your fish freeze. Without proper ventilation, many pond fish, particularly koi, are especially vulnerable to freezing if the pond ices over (or temperatures get ridiculously cold). By using multiple methods to prevent freezing, there is a higher chance your pond will remain warm.

TIP: If ice does form, do not break it by force! This will send sound waves through the water that can actually harm the fish. Instead, boil a kettle and put the boiling water in a pot. Hold the pot over the ice until a hole melts.

What to do with pond plants in winter

Begin by removing any dead or decaying plant matter and leaves from the pond. Then, cut back any yellowing / wilted foliage from the plants. Marginals that live nearer the top of the pond should be trimmed to surface level, or lowered deeper into the water, if possible.

Pond plants are very hardy and, in most cases, can be left in the pond over winter. Although some of the plants may die due to the cold, as long as the root of the plant is intact, it’ll survive through to spring. If any plant matter does wilt away over winter, it should be removed accordingly. 

When spring rolls around, they can then be individually assessed and re-potted into a larger basket if necessary, with aqua soil and given some fertiliser sticks. This will help with growth in the coming months.

Which pond plants will survive over winter?

Most hardy pond plants will go dormant over winter. They can be left in the pond and will gradually die back as the season changes. But don’t worry too much about this as, once it starts to warm up again, most pond plants will flourish as they did the year before.

Water lilies are a great example of this. They can be placed in the depths of the pond with little to no maintenance required over winter. All you would need to do is remove any pads that start to turn yellow so that they don’t decay in the pond. 

Marginal pond plants can be treated similarly. Make sure they are well submerged and trim them to the surface of the water level, taking care to remove any dead/dying foliage throughout the winter months. When it starts to warm up, they can be brought back up out of the water so they aren't fully submerged and allowed to grow above the water level. 

Should I cover my pond over winter?

Whether with a net or solid cover, there are many advantages to covering a pond, especially over winter.

A pond net can be used year-round as a defence system against predators. They can be fitted to most ponds easily and will stop decaying leaves and other debris from blowing into the pond. Decaying leaves can pose a threat to water quality and should be removed from the pond swiftly if they do get in.

A more solid cover, in particular, over the colder months, can be placed over the pond, which can act in the same way as a net but will also trap in the heat from the pond, thus creating a barrier between the pond water and the hot air above to keep the fish warm. 

The thicker the material used, the thicker the barrier. A range of materials can be used to make a pond cover such as tarpaulin, weatherproof foam panels, polycarbonate/corrugated plastic, bubble wrap & many more!

Do I need a pond heater?

Many people who keep koi and other pond fish do not heat their ponds at all. As long as measures are taken to ensure the pond is well insulated over winter, the fish can handle the temperatures just fine. 

Whilst a pond heater would keep the water at a more stable temperature over winter, most people do not choose this option as it can be costly, especially when used on larger ponds. Whether you’re using a heater or not, it’s always advisable to use a thermometer to track the temperature of your pond.

There are three main options if you do wish to add a heater to your pond:

  • Gas- fired Boiler
  • In-line Heater
  • Heat Pump

Gas-fired boilers with a heat exchanger work by heating water within the heating system in a gas boiler and then circulating it to a heat exchanger. Pond water is then pumped to the heat exchanger (in a separate chamber), which absorbs the heat from the hot water in the other chamber. It is then returned to the pond. 


In-line heaters work when water is pumped into the heating chamber where it comes into contact with the heating element. As the water passes through the heater, it warms the water before directing it back into the pond. 

Heat pumps are by far the best way of heating your Koi pond over Winter. Heat pumps work by passing air across a refrigerant which then heats incoming pond water in a heat exchanger. Using an air-source heat pump will significantly reduce your pond heating costs when compared with all other technologies.

If you do decide to use a pond heater over winter, we would advise that the pond should still emulate a drop in temperature (just not as severe). So, even with a heater, they should have a reading of 13-15°C to allow the fish to experience a change in season, even if artificial. 

By dropping to this temperature it will increase the oxygen levels in the water but will keep the fish in more comfortable conditions until the pond is back to normal operating temperature.

Caring for your fish over winter 

During the colder winter months, when pond temperatures drop below 8°C, our Koi Carp and Goldfish will change in behaviour as they go into an annual state of torpor. Their metabolism will slow down dramatically, resulting in our fish becoming lethargic with little appetite. 

To achieve success with your pond fish over winter, it is important to follow some basic principles. With a little thought and preparation, it is easy to succeed. 

We’ve answered some of the most frequently asked questions when it comes to caring for fish over the winter below. 

How deep do ponds need to be?

In areas with colder winters such as the UK, fishponds need a minimum water depth of 18” / 45cm. During freezing weather, fish will usually lie at the deepest part of the pond away from the ice, where the water will often be thermally stable at around 4°C. 

We can improve the safety of our fish in these temperatures by increasing the depth of the pond. While 18”/45cm is acceptable, 24”/60cm is better. With that in mind, we always suggest that our customers install ponds with the greatest depth they can achieve. 

Experienced Koi keepers will often build Koi ponds to a depth of 4’/122cm or more. This gives the larger fish plenty of room to rest safely during the winter months.

Do I need to undertake fish health checks during winter?

During the colder months, fish parasites are less prevalent, however, the UK is becoming accustomed to much milder winters. With that in mind, fish parasites may still be lurking in your pond! 

Given the fact Koi are much more vulnerable at colder temperatures, it is a great idea to have your fish checked for parasites before the weather turns too cold. This will give you ample time to treat the fish for any possible problems. 

It is best practice to get your Koi in excellent shape and free of parasites before winter. If your fish have parasites during the winter period, you’re setting yourself up for a fall. 

At Complete Koi, we undertake free fish mucous scrapes at the shop alongside free water tests. For more information on our services, click here.

Can I treat my fish for parasites in winter?

Many parasite treatments do not work at lower temperatures; therefore, your Koi must be treated for any possible parasite outbreaks before the water temperature drops to 12°C. 

If you do run into parasite problems during winter, the best solution is to heat your pond to a temperature suitable for treatments. 

Alternatively, you could move your fish indoors and into a holding tank where treatments can be used once the temperature allows. If neither of these solutions are possible or practical, then you could salt your pond to .3% and hope for the best! 

Do I need to check my water parameters in winter?

Absolutely! You should always check your water parameters and Winter is no exception. Unchecked leaf litter and decaying plant matter can cause pond water to become acidic which can be fatal to Koi. Additionally, decaying leaf litter can also contribute to the nitrogen cycle. 

In areas with soft water i.e. low mineral content such as Greater Manchester, water hardness should be checked frequently and GH and KH buffers added accordingly. This will ensure the pond remains at a steady pH of 7-8, which is safe for Koi and other pond fish.  

Biological filtration is less effective during the colder winter months. Bacterial colonies become less effective during colder temperatures and break down less ammonia and nitrite. Many pond keepers are guilty of overfeeding their fish and this can lead to ammonia and nitrite spikes if high protein feeds are used during winter. 

While fish can become overwhelmed by poor water quality, the effects of this are enhanced in winter. Therefore we recommend testing your water every few weeks during the winter months.

Can I add fish to my pond in winter?

In the colder winter months, it is generally not advisable to add new fish to your pond. Of course, this depends on your local climate. A more accurate way of deciding whether to add new fish would be when your water temperature is consistently higher than 12°C.


During winter, water temperatures (generally) gradually drop. As the water temperature decreases below 12°C the fish’s metabolism slows down and the fish eventually become dormant. When this happens, the fish will lose its appetite and usually lie motionless on the bottom of the pond. Fish in this state of hibernation can be vulnerable which is the main reason we wouldn’t recommend transporting fish in winter. 


If you take fish from a warm environment such as a shop and introduce them to your pond in Winter, the fish doesn’t have a chance to gradually adapt to the cooler temperatures. This is extremely stressful for ALL pond fish and will generally shock them, which can lead to unwell fish and fatalities.

It is possible to transport fish from one cold pond to another providing the temperatures (and water parameters) are similar. 

We still do not recommend this; however, if you must move fish during the winter months you should spend a little longer acclimatising them. Around 45 minutes would be recommended. Float the fish bag on top of the pond and periodically introduce small amounts of pond water to the bag to get the fish used to the different temperatures and water chemistry. 


As a guide, we would suggest purchasing fish for your pond between April and October. The best way of checking whether it is suitable to add fish to your pond is by investing in a pond thermometer. 

Can I feed my pond fish in winter?

Pond fish are cold-blooded (poikilothermic). Their body functions and behaviour are controlled by seasonal changes in temperature. At water temperatures below 10°C, Koi Carp and Goldfish experience a much slower metabolism and do not require much in the way of nutrition. Feeding pond fish high-protein meals in the winter can cause health issues as the fish simply cannot digest it. 


When your pond temperatures are below 10-15°C, we recommend feeding pond fish a low-protein diet such as Wheatgerm, or a Wheatgerm blend. In fact, it is a great idea to monitor your pond temperature using a pond thermometer and gradually reduce the amount of protein in their diet as we approach 10°C. 


When water temperatures drop below 10°C we should feed our fish exclusively on Wheatgerm. Below these temperatures, the fish will gradually lose interest in their food and may well stop eating entirely. Only give your fish as much as they are willing to eat. If the fish are leaving food uneaten, then you are feeding too much, and it is best to fish this out with a NET. 


If pond fish are fed a high protein diet over the winter months, there will be a build-up of excess waste, which the filters cannot process. Bacterial colonies living in POND FILTERS are less efficient in colder weather and the result will likely be Ammonia and Nitrite spikes which can be fatal to fish. 


Pond fish are extremely proficient at building up fat stores during their active summer months. They can use these energy stores to sustain themselves throughout the entirety of Winter. Do not fall into the trap of reverting to high-protein food on a mild Winter's Day. Wheatgerm is a natural food source for fish and is packed full of vitamins and minerals which will boost their immune system and aid digestion throughout Winter. 

As we emerge from Winter and pond temperatures rise above 10°C, we can begin mixing in small amounts of higher protein fish food with our Wheatgerm. As temperatures exceed 15°C we can begin to phase out the use of Wheatgerm entirely.

How much do I feed my pond fish in winter?

In winter, fish have a lower requirement for food. If the temperature is 10-15°C, we recommend feeding your fish a low protein feed such as Wheatgerm daily. Only feed them as much as they will consume in 3-5 minutes. If there is leftover food you are feeding too much. Remove the leftover food with a net and reduce the quantity next time you feed. 


When temperatures drop below 10°C, feedings should be reduced to once or twice weekly. Your fish are the best indicators in telling you if they need feeding. 

Introduce a small amount of food to the pond if you are unsure and monitor the reaction of the fish. If they are actively looking for food, then give them a little more. If they are disinterested, then remove the food from the pond. 

Fish build up incredible fat/energy stores over summer and do not need feeding during spells of cold weather. Leftover fish food can cause water quality issues if left unattended. 

Find everything you need for your pond this winter at Complete Koi

We hope you’ve found our blog on how to care for your pond and fish this winter useful. 

At Complete Koi, we offer a range of equipment required to ensure that your fish and pond stay happy and healthy this winter. Be sure to check out the rest of our website to find the best solutions to all of your aquatic needs. 

Shop pond equipment at Complete Koi today

For more information and advice on aquatics, check out the rest of our blog…

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