How to Keep Pond Fish Alive in Winter (Best Methods)

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Guide to Keeping Pond Fish Alive in Winter & Cold Weather Temps 2022

a fish pond starting to freeze over in winter
As pond water gets colder, fish start eating less and will eventually enter a torpor state; commonly called hibernation. Public domain.

Water temperature is a primary factor that influences both the health of the overall ecosystem and of your fish,  and so it’s important to keep an eye on this during the build-up to winter as temperatures begin to drop off. Simply put, cooler water is capable of holding more dissolved oxygen than warmer water, but water that is too cold also slows the bodily functions of most fish – which may or may not be desirable, based on the particular species.

Typically, keeping water below approximately 85°F (about 29°C) will support healthy levels of dissolved oxygen and it’s ideal to keep ponds below this temperature in warmer months. Furthermore, studies have found that increasing water temperature by even just a few degrees Celsius during colder weather is capable of vastly improving the metabolic rate and cellular respiration of fish, which in turn increases their consumption of dissolved oxygen and overall growth (which is why many koi breeders use heated tanks!).

multiple koi with a decreased appetite in winter
Fish eat much less in colder weather, but during torpor, not much food is needed as they use only tiny amounts of energy. Public domain.

On the other hand, water that is too cold can also negatively impact fish metabolism, growth rate, and reproduction. If water becomes too cold too quickly, many fish will go into shock and fall ill, which is why some pond owners in very harsh climates report mass fish deaths during sudden cold spells.

For many pond fish species, water that is too cold (below 50-60°F) will cause digestive enzymes to slow and possibly trigger fish hibernation, which could be dangerous if you have non-hardy fish in your pond. For tropical/non-hardy, breeds, which includes fancy goldfish, plecos, and guppies, they would need to be brought indoors well before temperatures get this low.

Can Koi & Goldfish Survive Winter In Outdoor Ponds?

many koi swimming in a pond during winter
Goldfish and koi can survive winter outdoors, so long as ponds don’t freeze completely solid. Public domain.

Of course, all fish have a particular temperature, oxygen, and pH range (all of which are related) within this that they do best in, so it’s essential that you do your research before stocking your pond so that you can be sure to have fish that will do best in the environment that you are able to provide and sustain. Goldfish, for instance, prefer temperatures between 68 and 72°F – around 50 to 55°F, they (and many other species like koi and orfe) will enter a state called torpor, in which they are inactive and reserve energy for survival.

Luckily, both goldfish and koi are able to overwinter successfully in outdoor ponds, so long as the water does not entirely freeze (they will congregate in the warmer water toward the bottom). With all of this in mind, you need to regularly monitor your water temperature throughout the year, ideally checking it the same time each day, and making sure to have some methods in place to help them overwinter more comfortably.

How Do Fish Naturally Survive Winter in Ponds & Lakes?

how to keep fish alive in winter
In much larger natural water bodies, fish retreat to deeper water or move to where water is calmer. Public domain.

In the wild, native fish have adapted to seasonal changes, developing a variety of ways to naturally survive winter. In freshwater systems with enough depth, warm water settles to the bottom (or hypolimnion) of the lake or pond during the winter due to its greater density, while cold water cycles up to the epilimnion (surface) and may freeze (this is called winter stratification). This is where hardier fish species, such as koi and goldfish, will gather to overwinter. As temperatures dip, so too does metabolism; fish will move about very little in an effort to conserve oxygen and energy. As cold-blooded creatures unable to regulate their own temperature, they will enter either torpor (which is essentially short term hibernation, lasting anywhere from a few hours to a few weeks) or full hibernation for the duration of winter until water reaches optimal temperatures again and their metabolisms resume.

trout and salmon are adapted to winter temperatures
Unlike koi and goldfish, wild species, such as trout and salmon, have adapted to very cold water conditions. Public domain.

Other fish may burrow partially or fully into the sediment at the bottom of the pond to keep warm, where they also enter a period of dormancy. Most fish will seek out still water, as any substantial current will sap away at warmth and energy. Less food will be consumed, both because metabolism has slowed and in an effort to keep it slowed to minimize energy and oxygen usage. Many are able to subsist on any algae present without much in the way of supplemental feeding. Still others are quite well adapted to cool waters, and will thrive throughout winter. Such species include sunfish, trout, and salmon, the latter two of which will even stay within the first few feet of the water’s surface despite colder temperatures. These well adapted fish often feed throughout winter, exploiting their prey’s reduced vegetative cover.

Many ornamental pond fish species, however, have been traded all over the world and are often not able to fully acclimatize to wherever they end up. Because of this, you may have to employ strategies like the ones detailed below to aid in their winter survival!

How to Help Pond Fish Survive Winter (Methods & Tips)

1) De-icers & Surface Heaters

pond deicer surface heater for ponds in winterTo allow for proper gas exchange during the winter, it’s imperative that you keep a hole in the ice. A de-icer is a disk shaped device that usually sits on top of the ice and will then melt a hole in it, keeping it melted for the duration of winter. Some de-icers can also be placed beneath the water’s surface, and will automatically turn on once the water begins to freeze. While easy to use, the downside of this method is the cost of running it.

air compressors keep water moving in winter and prevent ice formation2) Air Compressors

Air compressors are comparatively low-cost with high efficiency, facilitating water movement that increases the amount of oxygen in the water. These aerators also prevent ice from fully forming or weaken already existing ice until a hole or multiple holes form, also allowing for gas exchange. While more cost effective, some fish species may not find this method favorable, as they tend to avoid moving water in the winter. Make sure you do not purchase a too powerful compressor in comparison to pond size to keep from disturbing the water and fish too much.

3) Inline Heaters or Boilers

an in line water heater prevents ponds from freezing in winterAn inline heater or gas powered boiler can be used to prevent the water from freezing at all, kicking on only when the water falls below a particular temperature. Both of these involve passing the pond water through quite effective heating chambers in the devices and then pumping the warmed water back into the pond. If your water is already cooler than desired before you install or use either of these, you’ll want to warm the water up slowly (by about one degree per day) to minimize the risk of shocking your fish.

4) Cleaning Before Winter

vacuum clean ponds before winterAny organic debris, such as leaves, fish waste, and dead vegetation, present in the pond will use up dissolved oxygen as they decompose. In the winter, this is a particular danger as precious dissolved oxygen levels are already impaired due to ice and the loss of oxygenating vegetation. Before winter begins, you should thoroughly clean any muck and leaves from the pond’s bottom and change out at least 50% of the water, since cleaning will likely stir up sediment into the water.

5) Proper Feeding Habits

koi diet in winter wheat germ koi foodDuring cooler months, fish metabolisms will naturally slow and as such they don’t need to eat as much. Feeding too much in the winter often results in excess food floating about, diminishing water quality and dissolved oxygen as it breaks down. Decrease the protein content and increase the carbohydrate content of food as water temperatures fall below 60°F, since proteins are more difficult to digest. In addition, any excess foods will be excreted as ammonia, which is toxic to fish and can be lethal even at levels beneath 1 part per million. If the water temperature falls below 50°F, don’t feed your fish at all until spring. If necessary, they will be able to subsist off of any algae, insects, or plant matter present.

6) Move Fish (And Plants) Indoors

a water trough can be used to bring fish and plants indoors for winterSome fish species, particularly tropical, weak, or young fish, don’t do well at all with overwintering and should be moved to an indoor pond or aquarium where conditions are more regulated to suit them. This allows you to enjoy your fish year round with less risk of the cold damaging their health, though catching them and moving them may stress them out. Fish varieties that will likely need to be brought inside for winter include plecos, orandas, and black moors.

7) Increase Pond Depth

How deep should a koi pond be in winter? Koi and goldfish are hardy and typically capable of overwintering, but only if the water is greater than 4 feet in depth. This greater depth allows proper winter stratification to occur, thus preventing the entire pond from freezing and providing your fish with somewhere to hang out to escape the worst of the cold. If you live in a region with harsh winters, you’ll want to make sure that your pond is at least 4 feet deep or make it deeper if necessary.

8) Don’t Remove Ice and Snow

Leaving ice and snow atop your pond may seem counterintuitive to helping your fish overwinter, but these actually provide an insulating layer for your pond from harsh winds, and significant and/or swift temperature fluctuations. Though you’ll still need to create and maintain a hole or two to allow for gas exchange, if you live in a northern region you’ll want to keep most of the ice and snow to protect your fish. In southern areas, such as Florida, you may not have to concern yourself at all with overwintering, so long as water temperatures remain above 50°F.

1 thought on “How to Keep Pond Fish Alive in Winter (Best Methods)”

  1. Thank you for this information, Sadly I lost a couple of fish in the U.K. December freezing snap. Though having a floating disc to keep some water unfrozen, thick ice over the rest of the small pond deprived the water of oxygen. It was impossible to thaw a small patch because temperature were well below freezing night and day.


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