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The winter season for ponds can be a stressful experience for its inhabitants. It is important to prepare the pond as best as possible to minimize the chances of any problems arising. Cooler temperatures will slow the biological processes that occur in the pond - from fish digesting their food, to the efficiency of beneficial bacteria in biological filtration. Diseases also slow down during winter, but this does not necessarily mean fish are less prone to infection, just that pathogens and bacteria will take longer to spread in effect they work in slow motion. Cold temperatures can suppress the fish’s immune system if they have not been prepared properly. The stress of fluctuating and low temperatures can be a precursor to disease. Whilst we cannot control the seasons, there are things pond keepers can do to help prevent further stress on your fish.

How to Prepare Your Pond for Winter

In autumn, deciduous trees shed their leaves and many of these may end up in your pond if left untouched. Decaying leaf litter can acidify pond water and lower the pH and KH values over time. It is important to remove any leaves that fall in the pond by either using a net or a pond vacuum. Pond cover nets help stop leaves from reaching the water and also serve as a fantastic predator deterrent too. Once you have removed as many leaves as possible, we would recommend the addition of Sludge Remover which will aid in the natural digestion of sludge at the bottom of the pond. If added regularly throughout the season, it helps prevent any buildup of sludge leading into winter.

Pond plants also need some routine maintenance this time of year. Most pond plants are perennial (they regrow every spring and die back during winter.) Pond plants should be ‘dead headed’ once signs of die-back are observed. Water lilies are particularly important to cut back to the rhizome base. Your plants will then happily sit dormant through the winter months waiting for the spring when you can divide or re-pot if required.

The Big Freeze - Fish ‘Hibernation’ & Pond Heaters

When the cold weather arrives, the top of the pond water may begin to freeze over. During this time, water temperature averages 4°C and it is only then will ice form on the surface. At these temperatures, the fish enter their most dormant phase, known as torpor. Torpor is when an animal’s temperature lowers according to their environment and the availability of food. Hibernation, on the other hand, is a form of torpor, but is controlled by day length and hormonal changes rather than temperature. Koi and pond fish therefore do not hibernate during winter. When a fish is torpid, it does not need much food as its metabolism has slowed down to consume minimal energy. They will rest at the bottom of a pond, where the water is marginally warmer than at the surface and should be disturbed as little as possible.

There are two ways in which ponds are usually heated, both have different costs and benefits;

Option 1 is the more basic, cheaper and by far most common method and consists of a heating element usually around 150-300 watts (very similar to an aquarium heater) suspended from a floating ring of polystyrene. This method doesn’t actually heat the pond but prevents a small area around the heater from freezing over. This ice-free area allows for essential oxygen to enter the water and harmful gases like carbon dioxide to leave the water. In effect you are giving your fish a portal to the outside world allowing them to enter the torpor state without the fear of suffocation.

Option 2 involves an inline heater between the pump and filter, usually between 0.5 and 4 kilowatts (depending on the volume of your pond), which will actually heat the water in your pond allowing you to maintain a water temperature in excess of 12°C.  These heaters are usually powered by electricity but can also be powered via a heat exchanger with a gas boiler.  As these heaters prevent the water temperature falling, your fish can continue to feed and grow all year round with no need to enter the torpor state. This method is also useful if your fish have been infected with the carp pox virus as the symptoms of this virus are usually brought on in the spring and autumn by the fluctuating water temperatures. Consideration of cost should be given when considering this method, both of the initial outlay for the heater and the ongoing running costs.

If a pond heater is not a viable option, there are other ways to prevent or cure a frozen pond. For ease, many pond keepers will float a tennis ball on the surface. If the pond does completely freeze over, removing the ball will leave a small vent hole for gaseous exchange. If using this method, it is important to check the hole so that is doesn’t freeze over too. Ice should never be smashed. Shock waves from smashing ice penetrate down to the bottom of the pond where the fish are resting and can cause unnecessary stress. A safe way to create a vent hole if the pond has frozen is to place a hot saucepan on the ice until it has melted through to the water. It is not recommended to pour boiling water directly on the ice.

Feeding during Winter

As previously mentioned, fish enter a period of resting called torpor when temperatures fall. This lowers the fish’s metabolism, meaning they use very little energy in their bodies. It is not only a case of reducing the amount of food offered during the winter - the ingredients inside the food need to be considered too.


Pond fish will still be feeding between 8-10°C. It is important to feed the right foods for optimum conditioning before they enter their rest period. During this time (usually around October) it's recommended to introduce wheatgerm foods. Medikoi Wheatgerm with Garlic & Winter Pond Food both contain high levels of wheatgerm as a core ingredient. Wheatgerm foods are easier to digest and are lower in protein, usually 20-21%, compared with conventional fish feed which can be anywhere up to 50% protein. This is the ideal protein to fat ratio during winter months when the fish need less protein in their diet. Both Medikoi Wheatgerm with Garlic and Pond Winter Food also contain garlic, a natural appetite stimulant which should encourage your fish to feed.  Garlic has also been found to repel parasites which don’t seem to like the taste!

Medikoi Probiotic is another fantastic food that can be fed during the winter months. Its well- balanced, nutritional properties allows feeding down to 5°C, making an ideal companion food to Medikoi Wheatgerm to aid gut stimulation during the cooler months.


Winter foods should be offered twice daily and the amount offered should be judged on the water temperature and the activity of the fish. Only feed a quantity likely to be consumed within 10 minutes and remove any uneaten food after this time.  As the temperature drops and their metabolism slows, the amount of food fish will eat will decrease and the amount of food offered should reflect this. A floating pond thermometer is a handy tool as water temperatures are usually slightly different to air temperature. As the water reaches 7-8°C and below, the fish may stop feeding altogether.


Testing Water during Winter

Water testing is an essential part of fish keeping to monitor important parameters. The need to test during the winter is just as important as the summer months. Dangerous levels of ammonia and nitrite are just as likely to build up during the cold months. Leaf litter and dead plant matter after autumn can cause sudden pH crashes if hardness is also low. The PondLab 200 is a comprehensive testing kit that includes tests for all the important parameters, including ammonia, nitrite, hardness and pH.

Pond filtration will be less efficient during the winter months. Bacteria colonies become less active in low temperatures and this will lead to a reduction in ammonia and nitrite breakdown. Fish waste is, however, only one potential source of ammonia.  In autumn, many pond keepers continue their summer feed program of high protein diets. With cold filters running at a lower efficiency, it does not take much to overwhelm the bacteria and lead to ammonia spikes. Decomposing leaf litter also contributes to the nitrogen cycle as well as lowering pH. Hardness (KH and GH) levels will also naturally decrease over time. If KH levels get too low (below 6 dKH) the pH is vulnerable to crashing. Refer to the Water Quality in Freshwater article on the importance of keeping water parameters within recommended levels, and how to solve any issues.

Treating Ponds in Winter

Winter can be a stressful time for pond fish. The cooler temperatures reduce the immune system’s ability to fight off infection. Lowered metabolism also prolongs healing time. Many pond treatments will be recommended not to be used in low temperatures. This recommendation is usually based on reduced efficacy rather than safety as the efficacy of medications also decreases with lowered temperatures.  Treatments, such as Anti-Parasite & Fungus and Anti-Ulcer, Fin-Rot & Flukes, can safely be used, but may require multiple courses to eradicate a problem for this reason. Depending on the status of the disease, severe cases are best addressed to protect the welfare of the fish. Isolated or small bacterial or fungal cases are probably more effectively served in the following spring, but it pays to monitor the health of your fish during winter and be prepared to take action if required.

Many pond keepers will treat their pond in the autumn / early winter with an anti-parasite treatment as a preventative course. This will help lower any background levels of harmful parasites when the fish's immune system is struggling during the temperature fluctuations.

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