Overwintering Your Pond Fish

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If you’re planning on leaving your fish in a pond throughout the winter the video below will offer a few helpful tips that can make this an enjoyable time for you and a safe time for your fish.

As a teaser, if you’d like to see a guy in a Hawaiian shirt standing out in a snow storm (well ok, flurries) take a look. And you’ll find more helpful tips below the video itself.

If you have experience in overwintering your fish, please add your commentary at the bottom of this page, it will likely prove helpful to those just starting out in the pond care hobby.

[youtube width=”425″ height=”355″]http://www.youtube.com/watch?v=b966XBnRU0s&feature=youtube_gdata[/youtube]

Just to recap and add a bit on what was talked about in the video. As you go into the fall and winter you absolutely must get the pond in a fairly clean condition. Remove as much of the organic material as you can because as this material either dies or decomposes it will release unwanted elements in the pond water. These can build up and create problems that could prove toxic to fish.

We use several things that help with this. For mechanical removal we use some type of pond vacuum to remove gunk from the bottom of the pond. It’s helpful to maintain the use of an all season bacteria to keep nutrients and organic decomposition in check.

Pond leaf netting can help keep more unwanted material from going into the pond. Do your best to remove any material that falls into the pond itself.

Remove all tropical plants and move them inside if you want to keep them. Hardy plants should be cut back and placed deep in the pond or taken inside as well.

It’s advised to check things like ammonia levels with test strips about once a week throughout the winter, just to be sure this toxic substance is not building up in the pond. A 25% water change can be done going into winter, and even in the winter as long as the water temperature going in is fairly close to that of the pond water. Generally you want to avoid any major shifts (apart from nature) to avoid stress on the fish.

Ponds that are at least 18 to 24 inches deep should be able to safely keep fish over the winter months. The deeper the better is the general rule here. Use a pond heater or small pond aerator to keep a portion of the water open throughout the season. This helps to allow toxic gases to escape and oxygen to get in and helps to guard against the entire pond from freezing up. Using a hammer or other tool to physically open a hole in the ice should be avoided if possible.

Fish can be transitioned to a more carb based diet as fall approaches although some experts suggest you can simply feed whatever you’ve used throughout the year with good effect. As the water temperature cools, you’ll want to cut back on feeding the fish and as it nears 50 degrees F or so you’ll want to suspend all feeding until the coming spring.

Avoid the temptation, even with a short warm up of the weather, to feed your fish. If food is offered they very well may eat, but their digestive system is working so slowly that should the weather cool down again in a few days, the eaten food may stay in the fish and begin to rot…which obviously should be avoided. Pond owners have actually killed their fish in this way so it’s good information to keep in mind.

All mechanicals for the most part can be shut down or removed in the winter apart from the aforementioned heaters and aerators. Some folks will run their waterfall in the winter but many do not. Submersible pumps can be removed and placed inside in a bucket of water to keep their seals from drying out.

Building a covering, such as a plastic sheet, support by a wood frame, either flat or in a tent like structure can help keep the pond warmer and keep snow and ice from building up on it. Should snow cover the pond, this too provides some insulation against very cold weather and a pond can do fine in this natural state if a small hole can be maintained for air exchange.

So that’s a quick rundown on overwintering your fish in a pond. Have comments, questions, suggestions or tips? Just post those below.

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31 thoughts on “Overwintering Your Pond Fish”

  1. Hi, keep the snow in Iowa, not ready in Indiana yet. I have a 25X15X4 ft yard pond and have over wintered my koi, 25, 12 to 36 inches, in my pond for eight years.. I keep a punp going to move some water, 800 gph. The fish have done great over all. Have lost 2 and I feel that is a pretty good record. Thanks for your help and stories, always like to hear from pond folks. Happy winter and see my fish in the spring. Roger

  2. Thanks for the info on wintering…We run our pump (Waterfall) all winter. I keep a stock tank heater in both ponds along with an aerator. Seems to work well…Thanks again for the tips….Jan

  3. Thanks for the great info on wintering. I have a small pond (approx 70 gal) with a depth of 18 inches. As of now I have 9 gold fish, the largest about 6 inches, and one algae eater. Is this too many to winter in my pond?
    Temp here will be 50’s to low of mid 20’s for the Nov-Mar period. Again thanks for the great tips and info. Bob

  4. Hi Bob,
    Thanks for writing in. There are ratios of what might be best for fish stocking in ponds and one can follow these somewhat. I use them if someone has an algae problem to deal with.

    But without algae, and as long as ammonia loads stay down, every pond can hold different amounts of fish. The bottom line…it’s not a problem if it’s not a problem:)

    Keep an eye on things as suggested above, put a small aerator in there, maybe an aquarium type unit and things should go well for you.


  5. Hi Mark,

    While gone for a few wintery days, we lost all our fish except for three because our pump went out. Now we check our pump daily as a precautionary measure; in addition, since snow and freezing conditions are the norm in Salt Lake City, we purchased a trough heater too. No problems since then thankfully!

    Thanks Mark for your helpful videos–happy wintering!!

  6. I have a natural pond (50 x 40), fed by an artesian well. There is always a small space of open water and I have had goldfish in it for 4 years, getting huge! Last week a beaver appeared!!! Does anyone have suggestions on how to help him spend the winter then move on? I have been told trapping is not an option at this time of year…..

  7. Hi,
    I been wintweing my koi for 15 yrs. I keep all me pumps going. I do stop all water pick up from the bottom of the pond, in my case thats my deep end. The warmer water is in the deep so keep it there and pick up mid waters.

  8. Hi everyone: Lots of great information in Mark’s videos. His advice is right on. He has helped me greatly. I live in Southern Ontario Canada and no snow yet! Just lots of rain so keep the snow there. My pond is (15 X 20), has a 3 foot depth in deep end. I have had great success with my fish over the winters. I have only lost 2 koi over the years (I have 8)and have a lot of fan tail golds. I shut down the water fall, use aquarium pumps with air stones and use a pond deicer to keep the water open and a net of course. Again, Mark knows his stuff. Thanks Mark

  9. Hey Mark, In your latest video (which are extremely helpful) you mentioned shuting down your biological filters in the winter. What do I do with them? I’ve read that you shouldn’t clean them because then you have to start the whole beneficial bacteria process all over again. Is that true? Or does it matter? I have two, one purchased and one I made! Would it be okay to leave one running? I live on the coast in Va.and our winters are pretty mild, it does however, drop below freezing for short periods of time.

  10. Hi Jay,
    You’re right about the advice to not clean a biofilter, as this defeats the purpose of having the bacteria do the work of clean up.

    In most cases you’ll lose a lot of bacteria over the winter anyway with the cold weather. That and anytime water flow is shut down through a biofilter the amount of bacteria will be affected. So the best advice is to start fresh every year with some bacteria priming of the filter.

    With that said as long as you don’t get brutally cold you could try leaving them running and see how things go, it wouldn’t hurt a thing.

  11. we live in Ontario and it gets really cold . We keep a pump going all winter but it still freezes over the water going back into the pond . I go out and knock the ice off with an ax ( very carefully) to keep an opening in the ice . So far I’ve had no problems over wintering the fish . I’ve read were you can shock the fish by banging on the ice but i think that’s more myth than fact. Keep up the info videos Mark.

  12. Hi Jim,
    Thanks for your comment. I’m not sure it’s totally a myth. I think if you do it carefully all will likely be well. I suspect if things get too shook up at the very least it might stress the fish. I also follow the adage that if it ain’t broke, don’t fix it. Keep up the great work.

  13. Hello,
    Thanks for the information on wintering ponds. I have (3)three ponds of different sizes. I have over wintered my fish several years. I keep the pumps, waterfalls and filters running all winter. I also use UV lights in the summer. So far I have had great success in keeping my koi and gold fish. Some of the fish are the original ones from 15 years ago when I first installed a small 125 gallon pond. Keep up the good work, I enjoy your expertise. Thank you, sara

  14. Hey Mark – great video. This is my second season of over-wintering my fish here in the Seattle area. I have 1,000 gallon pond that is fed by a 80′ stream with falls at the end. There is a 4,800 GPH pump that feeds the falls and a pressure filter (the filter jumps returns water directly to the pond). I successfully over-wintered last winter (actually lost a few fish during the spring start-up which was baffling). Anyway, here is what I did and lessons learned…

    1) I netted the main pond to keep as much debris (i.e. leaves) out of the pond as possible.
    2) I kept the pump running 24/7/365 to keep the pond surface from freezing.
    3) Lesson – in late December we had 30+ inches of snow hit. This caused the water to leak out of the stream (snow acted like a sponge). At the time I no way to stop water flow to the falls only other than to turn the pump off. Not wanting to do that since I needed it feeding the pressure filter to keep the pond surface from freezing, I was out digging through the snow and installing a valve in may falls line. Pretty funny site at the time – but now I have valves in front of my falls and my pressure filter. With the falls/stream turned off, and a quick refill of the pond (almost 50% of the water was lost before I noticed the problem), the pond now stays nice throughout the winter.
    4) I used a monthly sludge bacteria formulated specifically for winter.

    Now for this upcoming season I’m already netted (due to a large heron finally finding the pond). Also this year since my pond is not that deep (maybe 24″ at the deepest), I cut a large plastic plant pot in half and sank them in the pond. This fish seem to like the safety of huddling under these (I’m not sure how long it will take them to recover from the heron terror – I’ve only seen 8 of the 14 fish I had).

    Anyway – already waiting for spring in Seattle!

  15. Hello,
    Unlike the previous comments, I experience little or NO winter. I live in lower Alabama, which rarely reaches 30 degrees.
    Do the same rules apply to my pond. I am a very NEW pond owner. I have only had my pond for 6 months, about 1800 gallons. I have a pressurized filter with a UV light. Also, I have a pump also that sits on the bottom. Did I understand that I need to turn it all off when cooler weather starts? Maybe this does not apply to me.
    I am experiencing terrible ALGEA (green water)
    Unfortunately, I cannot have the water hyacinth in my state. Any advice?

  16. Hi Kim,
    It sounds as though you will probably stay pretty moderate and as long as your temps stay up through most of the year, without extended periods of cold weather, you likely won’t have to shut things down. This will be a great learning process for you and don’t get too hard on yourself if things don’t always go perfectly.

    With all that said one of your most valuable resources will be pond owners in your own area. Those with experience can share with you what they do and the challenges they face and I can’t think of a better way to learn. Ask around for any local pond or koi clubs, or do an online search and you may find some help that way.

    For now, I wouldn’t worry about the green water as that should go away as the weather get’s cooler. As things warm up in the spring, try using a bacteria product and see if that helps a bit. Also try adjusting the flow rate through the uv. If this flow is slowed down some it may do a better job as they are specifically for green water types of algae.

  17. Hi Mark Thanks for all the advice.

    I want to move the fish into my garage for the winter.I bought a 200 gallon tank to keep them in. My pond temp is about 40 deg. I’m conserned about temp. swings. What happens if we get a warm spell that brings the water temp. back above 50? Will this hurt the fish?

    Thanks Again


  18. Hi Mark, I live just outside madison wisconsin and have had my pond for about a year and a half.The pond is 10’x12’x2′ with a small 3′ area. Last winter I had a really bad algae problem going into winter. The water was so green you could not see the fish at all. I shut down my waterfall and had a secondary pump directly in the pond to keep the pond ice open. and also bought a heater to use if the ice tried to close. as the pond started to freeze the fish came up into the shallows. I ended up removing all the fish except one ( had to rescue a few traped under the ice in the shallows or they would have been fish popsicles). I overwintered them in a tank in my garage. I am assuming the fish were coming up for air? The one fish left in the pond survived. This year the water is very clear as I added a UV clarifier. I want to over winter the fish in the pond if possible. would the algae have caused the oxygen problem I had or do I need to add a air stone? or should I be alright with just the pump? I also went from six koi/goldfish 6″ to 8″ too six koi/golfish 8″ to 12″ and about twenty baby goldfish now about 2″ to 3″ long. Would it be OK to overwinter some of the babies in a aquarium in the house? One last question, I have a large fantail goldfish that I have had in a aquarium for a few years. When we built the pond I put him in the pond. He was one of the fish I overwintered in the garage. The water only got down to about 45 degrees. will it be safe to winter him in the pond as the temps will be colder or should I bring him in with some of the babies?
    Thanks Patti

  19. Hi Brian,
    I’ll be talking more about bringing fish inside for the winter in the next installment so more details will be coming then, but to answer your question I believe you’d be ok. Ten degrees here and there is not a bad or dramatic shift and should be tolerated ok by the fish. You would of course see a wider swing in nature at times. In terms of feeding, I would still cut back to next to nothing on that temperature threshold unless your constantly above 50 to 55 degrees.

    Hi Patti,
    I certainly don’t see a problem with bringing some of your fish in if you want to. As for yoru older fish I would think it would be fine as long as your able follow some of the recommendations above.

    With that said, if you’re concerned in anyway, bring him inside. I commend you for being proactive last year with the fish and being prepared to adjust your plan like you did. It’s possible that the green water algae did pull oxygen from the water as it died off. Any plant will do that during die off so it’s good to get as much of those out of the pond before they start declining.

  20. I have a 300gal pond with 2 koi and 3 goldfish located just north of Chicago. I have successfully wintered my fish for 10 years. What I do is disconnect the hose from my 200gph pump and put the pump on one of the shelves of the pond. This keeps an open hole in the ice into the beginning of January. I also install a deicer at the first sign of ice. It is very hard to get the deicer in the pond when there is an inch or two of ice on it. I only plug in the deicer when it is really cold in January and only until there is an opening. This cuts down on your electrical bill.

    I have also found it very helpful to put a small garbage pail over my electrical plugs so that when the snow comes it is easier to find and work with them.

    Great videos Mark! Keep up the good work.

  21. Lincoln, Nebraska. We have about a 3000 gallon pond with a depth of about 4′ at the deepest. For the past several years we have left our waterfall running with a submersible pump and have used a stock tank warmer to keep the surface open. It keeps about a 10″ diameter opening. The waterfall also keeps an area of the pond open for all but about 2 weeks during the winter. We have stopped feeding the fish several weeks ago. Trimming back all the follage around the pond is important to make sure it does not get into the pond and decay over winter. But all the work is worth it.

  22. Hi Mark,

    Thank you for the video, it’s the first one I’ve seen and hope to see more. Also, the questions and comments have been great also.

    I’ve had fish for almost 10 years and it’s all been trial and error. I have expanded my pond twice, because of course, you always end up getting more fish than you start with. I kept them in wading pools during the expansion with small pumps and they all came through like troopers.

    My pond is now aprox. 11.5′ x 6′ x 2′ deep, so just over 1,000 gal., with a 2’x6’x1′ ledge with pea gravel surrounding my pump and filter system. It holds six 12″ to 20+” koi, maybe four goldfish, and five shubunkins.

    Two of the koi I’ve had almost from the beginning of first pond and they have come through every winter just great, as have the rest of the koi the last few winters. One shubunkin, for the last few years and we’ll see how the 4 new one’s survive their first winter.

    I do nothing special, keeping the pump running all winter, and removing the leaves that fall. Only this last winter did we remove any ice and the waterfall kept an open space anyway.

    I guess if you live in the PNW like I do, you seem to have an easier time of it.

  23. Hi: I live in Nova Scotia, Canada and in towns we have limits on the depths of our ponds. In Amherst, it is under 20 inches and the winters can be brutle here. I have wintered several time in the outdoors, but it is risky business. If you get the cold before the snow then you will loose your fish. Several years ago I lost all my fish. The next year I had some that were freezer burnt and they eventually died so I have taken a better route I winter my fish in my garage which is unheated. I try to get most of the fish out but sometimes I miss a couple, and in the spring they come for food as soon as they see me out there getting the pond ready for the rest of their mates. The first year I did this I had major ice buildup in my pool and was constantly adding warm water, so my husband insulated the door that was close to the pool and it helped 90%. The next year I put foam insulation under the pool. I have not lost any since I started doing this. What I did was buy a 30″ deep x 10 foot wide kiddie pool and I fill it with water and put a small pump with water feature in the centre. I bring in my fish (60 or so, pending on the number of babies I have. This year I only have four babies and last year I had about a dozen)late October and they stay there until mid to late April, pending on the ice in my pond. I wait until it is all melted and starting to warm up. When I first bring in the fish, until late December I have to add ice cubes to keep the water cool enough and after that I have to keep a check on the buildup of ice as once there is a little forming on the pump, several hours later you have an ice attraction. It is very important to keep the water cool enough so you do not have to feed the fish in the house. Cleaning the pool can be very difficult in the house so I try my best to keep it at a steady rate of 32-45 degrees. Seems to work well for me. A note of interest, last winter I had four frogs winter with my fish. They did very well and I did not have to feed them either. This is what works best for me in our extremely cold climate and I think if you live in a cold climate like ours, this solution to winterizing your fish can be very useful.

  24. Sheila…thank you!

    This is a great post and wonderful advice from someone who has to deal with extreme winter conditions at times.

  25. Another superb video …
    I do like this site !
    as usual concise and to the point …
    easy to understand …
    I prefer heating to 16.2C that way the thermostat kicks in at 15.7C
    but I understand for 1 reason or another not everyone can heat …
    It’s not the cold which kills Koi …
    it’s the fluctuating temperatures in Autumn Fall and especially Springtime …
    the Parasites and Pathogens thrive and multiply far better as the temperature begins to rise, whilst the media bacteria is still dormant ! and the Koi immune system at its lowest level ! and of course the dreaded temptation to feed when too cold !!! Koi do NOT have a stomach, just a crude gut, and as pointed out in the video the food will just rot inside a Koi gut ! causing Bacterial disease and Ammonia inside as the food decays !
    I have a silk shirt very similar Mark …
    but I’m only brave enough to wear on vacation !
    Congratulations once again Mark, on a superb Website !

  26. Hi Peter,
    Many thanks for the kind words and for the timeless advice. It’s spot on. And may I say that “bravery” is often not a word related to wearing a silk shirt in a snow storm. Other words might best describe that:)

  27. I live in southern Alberta Canada,last winter was the first time to leave my pond running and keep fish outside.I have 5 gold fish and 3 koi 4 to 6 inches,pond size 8’x14′ and 20″-30″ in deep.pressure flo filter,900tgph submersible pump.the waterfall is about 5′ tall.mid oct temp dropped below 0 degress C ,after two days had to shut down waterfall because water started to be rerouted beside the pond due to ice. So I rerouted my waterfall diffuser around the side of the fall,and placed it to empty 4″ above water surface.high flow creating lots of turbulance will keep a hole in the ice and helps aerate pond.

    This worked well till the temp dropped to -12C to -15C overnight 5 days in row.Then my 1 1/2″ hose froze up along with the filter(yes it cracked the filter).I broke a hole in ice above pump and disconnected the outlethose and aimed the pump toward the edge of waterall to get high cicular flow around the edge of pond (kind of like river).i broke a hole in ice 6-8″ two to five times a day useing sledgehammer.If you use the same hole or mantain the same the ice never get more than 1″ thick so you really do not have to swing hammer hard(10lbs sledgehammer makes perfect size hole with little effort,remember the more often you break hole open in a day the easier it is and if your not bashing like a mad man you won’t stress fish.5 times a day at temp of -35C,at this temp the hole will freeze over in a hour or so 2hours 1/2 of ice will form).

    I stopped feeding mid oct and did not feed again till erly may.lost 1 koi i think it tried to hide in rocks near surface got stuck in ice and froze. this year i’m adding an air stone.pond foze over last week i’ll see how it goes. have at laest 50 babies that suvived the summer now 1″-2″ in size.How many will survive winter?

  28. Hi Danny – It will be interesting to see how many make it through successfully. Best of luck this winter to you and all of them!


  30. Hey,

    I found your video helpful but just have a few questions specific to my pond. I have a 125 gallon pond and am living in a zone 5. I have bought a floating de-icer for the pond, however, have 5 koi in there and am wondering whether or not to leave the pump in. I seem to be getting mixed messages and nothing specific to my size of a pond.

  31. I leave a bubbler on all winter ..not a small one It works for me ..About $ 70.00 ..I live in NY ..I put in two heaters that melt ice in the pond…when it freezes over I boil some pots of water ..be careful not to touch the rubber liner leave the pot on top of the ice till it melts a hole for the gases to escape, you can repeat this until there is a melted hole in the ice.My Pond is 3 ft deep and my Koi are about 18 to 20 inches long.Hope this helps… Liz

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