Pond Fish Care in Winter

Pond Fish Care in Winter

Winter isn’t the most exciting time of the year to be a water gardener, but it’s one of the most important times for maintenance.  Starting in fall, the sunny summer weather slowly starts to cool as the leaves change colors.  Water quality is still the main goal, but instead of fighting algae, the focus turns to winter preparation.  During the coldest part of winter, many ponds are shut down until spring.  Koi, goldfish, and other pond life are often a water gardener’s favorite part of their aquatic oasis, and their lively activity is missed during the winter.  In a deep enough pond, fish are able to spend the winter at the bottom of the pond until the water warms up in the spring, but require a little help.


What are my fish up to?

Fish are cold blooded creatures; they do not generate their own body heat.  As the water temperature drops, fish’s metabolisms and bodies begin to slow down.  They require less food and become sluggish.  Once the water temperature drops below 50°F, their bodily functions nearly stop completely, similar to hibernation.  They use any excess fat on their bodies for energy and swim to the bottom of the pond and hang out with their friends.  The water in the deepest of the pond is actually the warmest.  All of the pond life will stay there until the spring.  



If your pond is at least 18 inches deep, depending on where you live, your fish should be fine to stay in the pond all winter long.  The pond needs to be deep enough that the water will not freeze all of the way through.  The fish need enough space to swim at the bottom of the pond without being too crowded.  


If your pond is too shallow, you can move the fish inside for the winter.  The fish should be kept in a climate controlled area.  There will typically be less water in the temporary home than the pond, and the fish are at risk for freezing.  The holding container should be large enough to keep the fish from being too crowded.  If the container is shallow, you can use pond netting to prevent the fish from jumping out.  The fish will need oxygenation, either an aerator or pump.


Fish Feeding

Once the weather drops to the 60s and 70s consistently in the fall, fish feeding should slow down.  Fish digestive tracts start to slow with the dropping temperatures.  When the water temperature falls below 50 degrees, their systems come nearly to a halt, and should not be fed.  Their bodies cannot digest the food, and they can become sick.  The uneaten food can also cause poor water quality.  This pond fish feeding guide will give you step by step directions on how to feed your fish in all temperatures.



Pumps, fountains, and waterfalls can also cause overcooling, and hurt the pond life.  The surface water of the pond is the coldest during the winter, and the deepest sections in the bottom have the warmest water.  Pumps, fountains, and waterfalls mix the water when running, which is great for normal operation, but they mix the cold and warm water too quickly in the winter.  The overall pond temperature drops, causing overcooling.  The fish can become stressed or even harmed.  That is why it is best to removed, cleaned, and maintained pumps, fountains, and waterfalls once the water begins to freeze.


Surface Freezing

Once the temperatures drop enough, the pond’s surface water freezes.  If there are no openings in the surface ice, the water can not oxygenate.  The fish still need sufficient oxygen levels, even though they are less active.  The lack of oxygen is one of the main reasons for a winter fish kill.  


Aerators and de-icers are great for keeping a small portion of the surface unfrozen.  Aerators should be placed in a shallow area of the water to prevent overcooling.  Aerators diffuse oxygen directly into the water.  As the bubbles rise to the surface, the water moves and agitates.  The moving water keeps the surface from forming ice.  Aerators may not work in severe winters, and can be used along with an de-icer.  De-icers use heat to melt an area of the surface ice.  It is not enough heat to melt the entire surface or warm the water, but it is effective for keeping a small hole active.  


If you need to quickly create an opening in the ice for pond maintenance, do not use force.  Using a drill, ice saw, or an ice pick to cut an opening into the ice creates shock waves that stresses and can even kill the fish.  Instead, you can use boiling water to create an opening in the ice without distressing the pond life.  


Water Quality

During the fall it’s important to keep your pond as clean as possible.  Debris that settles to the bottom of the pond during the fall begins to break down in the winter.  Organic waste like leaves, uneaten fish food, lawn clippings, etc. releases gases as they are broken down in the water.  The frozen surface doesn’t allow for natural oxygenation, so the gases have no place to escape.  They become trapped and can be harmful to pond life.  Before the surface freezes, remove any debris that falls into the pond water before it settles to the bottom.


Things to watch out for:

Sick Fish

A fish’s immune systems also slows down during the winter, making them more vulnerable to parasites and diseases.  Before you close down your pond, check your fish for any signs of sores or parasites.  Determine if the whole pond needs to be treated, or just a few fish.  If the illness is isolated, the sick fish can be removed and treated in a separate container.  


Snow Fall

While falling snow is beautiful, it can be problematic for your pond.  Snow buildup on the surface can prevent light from penetrating water.  The little bit of light the pond would normally receive helps oxygenate the water.  The pond plants produce oxygen through photosynthesis.  Removing snow also helps keep the pond visible so kids and wildlife don’t accidentally fall into the water.



Fish are more vulnerable during the winter, as they move more slowly.  They are not able to dart out of the way as they would in warm water.  Fish also have less room to swim around, and huddle together in warm water pockets in the water, attracting predators.  Structures like koi castle or pond netting can help protect fish from predators.  


Power Outage

During a bad storm, the power may be affected.  Power outages can last only a few hours or sometimes days.  The aerator and de-icer won’t be able to run without power, and the surface of the pond will freeze over if given enough time.  It may be unsafe to check on your pond during the storm and pour boiling water over the opening.  An uninterrupted power supply (UPS) or generator will be able to give the pond power temporarily until the power is restored.


Pond fish care in the winter helps make the transition into spring easier.  Fish will be healthier if they can be kept relatively stress free and happy during the cold winter.  If your pond is deep enough, your fish will be safe spending the winter in the pond.  They will be fairly inactive and go into a hibernative state, but will be ready to start feeding and greeting again when the weather warms up.  Maintaining water quality is very important for fish health and general pond health for the spring.  Most “winter kills” are caused from poor oxygenation or shock from overcooling.  After a little help, your fish will be able to go back to their normal habits and schedule in the spring, ready for a new year of excitement and water gardening.


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