5 Ways to Protect Koi Fish During Freezing Weather

A koi fish pond is a beautiful addition for any home. But you want to keep those fish alive, especially during freezing temperatures.

Ice by itself is not dangerous to koi, as long as your pond is deep enough not to freeze completely (3-5 feet or more).

The danger to fish comes with the reduced oxygen and increased toxic gases caused by the sealed over water surface.

While you could move your fish and plants to a temporary indoor tank for the winter, or install pond heaters that use a lot of energy, in many cases you can overwinter your fish outdoors with a little care. Keep in mind these tips:

1. Aeration: Have an aerator running in your pond at all times to keep the water stirred and promote the exchange of gases. An aerator is a lot more economical than a pond heater, and it will oxygenate the water while reducing ice build up.

2. Ventilation: If you’re aerating the water, all you need is a small hole or two for the bubbles to escape. You don’t need to remove the rest of the ice from the surface.

3. Balance: Koi (and many other fish) go into a sort of dormancy in the winter, using less oxygen and naturally settling to the bottom of the pond where the water is warmer. Your aerator should gently stir the water without upsetting this natural balance. Go for a steady stream of bubbles, not a rolling “boil” or strong current. If your pond is 4-5 feet or deeper, put the diffuser at the middle of the depth range (not the deepest part) so that the fish can retreat to less-disturbed deeper waters if needed.

4. Ice: Don’t break the ice in your pond by hitting it, since the force can cause shock waves that may hurt the fish. Keep an eye on your ventilation hole and remove any thin ice before it becomes thick and hard. If necessary, you can sit a full, hot teakettle on top of the ice until it melts a hole. Otherwise, let it snow! The snow, ice, and surrounding earth will help insulate your pond from the frigid air.

5. De-Icer: If your pond threatens to freeze completely, you may need to install a de-icer to keep at least part of the water above freezing. Your fish will need a few feet of liquid water in the pond throughout the winter.

Further Information

  • Overwintering Koi Under Thick Ice (pondtrademag.com)
  • Do I Really Need a Pond De-Icer? (thepondguy.com)
  • Over-wintering Your Pond and Water Garden (pondsolutions.com)

28 COMMENTS

  1. HI There

    I have a koi pond that has a waterfall (which we turn off during the winter) and the pond is deep enough that we dont have to bring our 10 koi inside. We also use a bubbler pump during the winter but we still have a hard time keeping the pond from freezing over we had to build a cube around it which helps but it still freezes over (well it forms like a ice globe over the bubbler) is there anything else we can do to stop it from freezing over??? should we build snow hills against our cube (made out of thick styafoam) or what?>

    [email protected]!

  2. Hi. I seem to be having a problem with my heater for my koi pond. It did the same thing last year so I went out and bought a new one. It’s doing the same thing. It goes red for a while when it gets cold but now it just stays green. Why is this happening? Is there something I the water that’s affecting my heater? Is it some thing to do with my electrical outlet.

  3. Thank you all for these tips, I have a pond with goldfish in and it is my first year. Now I know about frost and think I can protect them what do I do about feeding/not feeding them? I heard that they don’t need feeding in the winter but when and how do you stop?
    Thank you in advance for any replies

  4. Your fish can retreat to less-disturbed deeper waters if needed. And always use for repair a sealant which is not harmful for waters plant and fish.

  5. I live in a climate which summers 100F winter -40F . my pond is 1000 gal ranging from 2 ft to 4.5 ft deep.

    I use a bubbler for an aquarium in the deep end and suspend above it and 4 inches or so into water a pig water trough heater, about 60watts. this keeps ice from forming over the bubbles. Works for last 4 years.

  6. Hi. I live in edmonton alberta, Canada. I have a pond with Koi & goldfish, which i leave in the pond in winter. Just want to know, what other kinds of fish will survive our cold winters?

  7. I have a koi pond. This is my first year. It’s 2 1/2 feet in the ground. Is it better to winter outside or bring them inside? Thanks

  8. I have 100 gallon pond my 2 koi died this winter.I ordered a heater for next year
    I have a waterfall in there and a pump. I left my floater plants on durring winter to give cover that was a mistake from what I’m reading.is a floating heater best bet for next year? I believe pond is 2 foot. Lana

  9. I have 350 gal pond with aprox. 2′ deep. got 6 Koi at the age of 4″ in length. and 4 Gold Fish at the age of 2″ length. I live in Virginia and we dont get that much of snow like NY or Chicago. But we get snow storm, Do I need to take them inside or leave them in pond with Aerator or Bubbler, De-Icer.. Do I have to leave my waterfalls countinue running during winter?? Plz. HELP.!!… Tnx in Advance.. :))

  10. THE SWIMMING POOL!…Here’s an idea for you all. In the fall I drain all the water from my KOI pond, net my KOI, and toss them into my above ground, 4ft deep, swimming pool for the winter.

    This works great as all my KOI have survived the ice cold Connecticut winters. Just be sure all the chlorine has dissipated before throwing them in the pool. Works great! Try it!

  11. I live in NYS and I have a fish pond with koi and gold fish. During the winter we have two airation and a homemade pond heater I found on utube to make. Everything was working great until late February where we had some cold and warm days. The pond became green and four of our big Koi ( 1 -2 feet long ) I found dead just today (4/2/17). Does dead fish contaminate the water?

  12. When I lived in Alabama we would occasionally get a cold snap where it would freeze an inch or so of ice on the top of the pond. I kept a basic fish tank air pump bubbling air in the pond that was about 2 ft deep. Also, because of the cold, I usually only fed them once a week, compared to the usual daily feeding I did during the summer. Never lost a fish due to the cold.

  13. First year Boston winter for Koi (8)and Goldfish (13) pond approx 2-3 feet deep/4,000 gallons. Also have a stream w/waterfall.
    I intend to keep everything flowing. I will watch out for ice along stream. Should I buy an aerator or heater?
    Also, when do I stop feeding Koi and Goldfish?

  14. I have a koa pond 25x 8 feet and six feet deep.my waterfall keeps a good movement of the water and want to know if this will bother fish during winter. I live in British Columbia and at some point the pond will freeze for usually a month

  15. i have 3 gold fish and 1 koi about 5 inch long it gets down to 35 degrees here i Hemet Calif. i have a water fall and and flowing water under the water in the pond it has s no plants i have lava rocks. can the fish live in this weather ? they dont eat much in the winter i know. would a cover planter help them to hide in help ? it’s a 450 gal tank.

  16. What is proper Koi pond upkeep in the winter in Arizona? My fish are not coming up to eat, started a week or so ago when it started to get cold out. Thank you!

  17. Excellent tips!! These are very beneficial and useful tips for me. I didn’t know How to Keep Koi Fish in a Pond During Freezing Weather. Thanks for sharing this informative information.

  18. All of my koi died this year. Pond is three and one half deep. Had a bubbler and heater
    We’re in Southwest Michigan and had the coldest strech that I can remember. Three weeks of weather in the teens. But we did have a good blanket of snow. Off course the gold fish survived

  19. I have someone that helps with my koi pond. He instructed us to stop feeding in Novemeber and resume in March. Is this true?

  20. Hi,
    I have a natural pond in CT. It is about 1,500,000 gallons and goes
    down almost 12’. I have probably 40 large koi and 200 goldfish. It is late October and I noticed there are several new baby koi. They vary from less than an inch to 2 inches. Will they survive the winter? I can try to take them out and bring them inside, where I keep a large, well
    filtered tank.
    Thank you!

  21. I am completing my first koi pond with waterfall. It is 8’ x 11’ and 3 1/2’ deep at the deepest end. I live in Northern California (Sacramento area) and want to know if I can purchase and add koi to my pond just before Thanksgiving. My 15’ doughboy pool does not freeze and we may have only 3 to 7 days of air temps going below 40 degrees. Do not want to kill new koi. I do have a airateor installed with two 4” air stones. Please let me know your expert opinion since I am brand new to ponds and koi. Thanks and have a great day.

  22. I live in northern Michigan and have a 1500 gallon pond 3 ft deep. To winter my fish I use a 5 gallon bucket with a 1000 gph pump extending 3/4 inch pvc upwards and above the surface of the ice then returning back into the ice using several 45 degree fittings. The exposed pipe I drilled a 5/16 hole and used an ink pen casing cut at. 45 to create a Venturi which adds bubbling air to the water. I have found even if the pond feezes over the air still escapes at edges and at what I call Swiss cheese ice with air pockets . Gases still escape just fine. I’m 5 years doing this loosing 1 female which probably had decaying eggs in her. This is common in females overwintering. I’m not a pro by any means but this works for me. We get below zero at night for weeks at a time and I get about 8 inches of ice. My koi eat nothing from late October to about mother’s day. I imagine they snack on some algae but not much. When the water is over 55 I feed once a day until mid June by then the water is over 65 and let the buffet start. My pigs eat 4 times a day. I malchite green my water once a month to keep microbes away starting in March through October. That my regimine that works in my neck of the woods. Hope this helps a koi enthusiast that lives in a cold climate.

  23. I was lucky I live in non four seasons weather I have no problem in dealing with season changes in koi pond maintenance

  24. Hi there, I’ve had my Koi and goldfish now for 2 winters. Previously they were in a stock deep at about 2 1/2 feet deep. Now that he is big, I switched to a 200 gallon pond. The problem is, it’s only about 1 foot to 1 1/2 feet deep. I’m worried about this upcoming winter. I’m in Washington state and we have a solid month of temperatures below 32 degrees, sometimes in the negatives. I’m thinking of investing in a de-icer, putting insulation around the base of the pond, and building a green house sort of structure over the pond. I’ll keep my pump and aerators going through winter. Do you all think this will be enough to keep my Koi and goldies alive, or will the 1 foot pond completely freeze regardless of these efforts? Everything I’ve read says that a pond should be 3 to 5 feet deep to avoid a complete freeze. Help!

    Thanks!

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