Overwintering Pond Fish Indoors

Published on January 1, 2013

Just how cold can it get in some parts of the Continental United States? Does it get cold enough to freeze ponds solid?

It can get very cold, as low as -30 to -35°F. And yes, it can get cold enough to freeze some ponds solid. Small ponds and above-ground ponds are especially at risk of this happening. Larger, in-ground outdoor ponds of, say, 1,000 gallons that are at least 2.5 feet deep can successfully overwinter hardy fish without supplemental heat during these severe conditions. The outdoor techniques outlined in that article have worked for overwintering both koi and hardy goldfish in my own ponds as well as those of my clients for over 20 years.

There are people with large in-ground ponds who successfully overwinter all their fish in certain winters, while in other winters they lose some or all of their fish. How can this be?

This is due to all the variables that each pond experiences going into and during the winter, giving each pond its own uniqueness. The severity of the particular winter, the amount of organic matter in the pond, the number of fish and/or increase in size of fish over the years and the health of the fish going into winter are the proven variables. Some other variables are more theory than fact. Stay tuned for a future article on this subject.

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 This article is for those people who have lost fish attempting to overwinter them outside which may be due to the above variables. It also is for people who have those small ponds in areas where the temperatures can get to 10°F. or colder. For all of these ponders there are three other options for overwintering fish. They are: adding supplemental heat, building a greenhouse-like structure over the pond, or moving the fish indoors.

Supplying supplemental heat to an outdoor pond with a stock tank heater can be very successful, but it is imperative that aeration is also used. For mild winters the heat addition alone **may** be enough to overwinter the fish, but in more severe winters both heat and aeration are necessary. Always use aeration with the heater – it is too risky not to. Aeration that is too heavy in small ponds can create a current that the fish will have to fight all winter. They simply do not have that much energy during the winter and can die. In these situations, all that is needed is a good stream of bubbles located in the center and on the bottom of the pond.

goldfish gallon bucket
The process of acclimating fish to an inside pond (tub) is shown here. Small hardy goldfish have been placed in a 5-gallon bucket in their outdoor pond water.

There are a variety of heaters but, for the extreme cold we are talking about, a 1500-watt floating stock tank type of heater is necessary. The down side of this option is that the electrical costs can bust the budget. These costs could be as high as $50/month, or more. Of course, there are even bigger heaters that can really chew up a budget. My experience tells me that, unless you provide some kind of insulation around the pond, you literally are “spitting into the wind” with these heaters. The heat loss is extremely fast on a zero-degree day.

Building a greenhouse or other type of structure over the pond is another option. However, this can be a huge cost and a big undertaking. Most will not want to do this. Or “This will simply not be an option for most people.”

>> Pond Life | Benefits of Adding an Avian Element to a Pondscape

Due to these high costs, moving fish into an inside pond is the best option. So, in this article we will discuss the techniques for overwintering pond fish indoors.

Even with larger ponds, all warm water fish (tropical fish) and the more exotic breeds of goldfish that are not quite as hardy will need to be moved into warmer inside conditions. Most goldfish that have a body that is egg-shaped, and the trilobed-tailed goldfish, are not as hardy. I have tried to overwinter some of these goldfish varieties outside and have not had success during the more severe winters. Success can be achieved when the winters are milder and/or supplemental heat is used. But it is a gamble to risk these valuable fish in such an attempt. Moving them out of the brutality of winter and into an indoor situation is a necessary step.

**Tropical fish need to be moved inside before the water temperature goes below 65°F**. Any colder and a lot of them will die – they simply cannot take it. All of our more hardy cold water fish can be moved at any time before the pond develops permanent ice. This is what I call “freeze-up.”

This structure is built over a pond in the fall to provide protection from the winter winds and snows. Under this greenhouse the temperature of this pond stays about 50°F. in midwinter. Most people will not want to go to this much expense and effort to overwinter their fish. Photo by Larry Thompson.

Moving the fish inside can be very hard on the fish, even to the point of losing them if not done right. Before the fish are caught, have the indoor pond ready for the fish transfer. If it is possible, move a portion of the outdoor pond water into the inside pond. Ideally, 20 to 50% of the volume of the inside pond should consist of water from the outdoor pond. This is not absolutely necessary, but it is less stressful for your fish if it can be done. Usually the water temperatures that the fish are in while outside will be colder than the water they are going into. So, the process of acclimating them to the indoor pond water’s temperature is critical and should be done **slowwwwly**. I like to do this over a period of several hours if the temperature difference is large. If the difference is less than 10 to 15°F, then an acclimation time of only an hour or so should be sufficient.

When acclimating the fish, it is important to acclimate them to the water chemistry in addition to temperature. To do this, start by placing the fish in a container to move them inside. Have this container filled with just enough of their outdoor pond water to allow for more water additions. Then, add 10% indoor pond water to the transfer container containing the fish, wait 15 minutes and then add another 20%, wait another 15 minutes and so on, gradually increasing the amount of indoor pond water each time. These wait periods are longer when the temperature difference is large. This container should be aerated and covered during this process.

Outdoor fish are used to swimming in a much larger pond and, when restricted to a smaller container or pond, they will jump, with possible tragic consequences. So, covering the transfer container is important during the acclimation process, and covering the actual indoor pond with a jump-proof type of cover is equally important. Choose a type of cover that will keep a fish in the container and take a fish hitting it without becoming dislodged. Sheets of flexible plastic or tarps are examples of covers that are not always jump proof and should not be used. A fish can flip those off. Use heavy welded wire, hardware cloth (wire), Plexiglas, or glass. Even with these types of covers you may add weight on top of them to insure that they stay in place even if a large fish hits it, attempting to jump out.

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The size of the indoor pond is always a critical factor in fish survival – usually, the bigger the better. The larger volume allows for more of a fish waste “sink” before a water change needs to be done. Large fish also need a correspondingly larger container or pond. Large koi need to be able to easily turn around in their pond. Too small of a container can risk the health of your fish.

With hardy fish, attempt to maintain the indoor pond water as cool as possible without the water freezing. An attached garage or a cool basement is usually a good choice. If the fish are displayed, upstairs, in the living quarters, then the pond/aquarium will be warmer, which is fine. However, the warmer water dictates a couple of additional requirements. An even larger container should be provided, because the fish will be eating more and therefore excreting more. Again, larger volume allows more of a fish waste sink. Even with a larger container, it is vital that more water changes be made.

Aeration and circulation is critical to the health of the fish in any pond, but it is especially true for indoor ponds. The amount of biological filtration you provide depends on how much the fish are fed, therefore more fish waste is produced. These bio filters will break down the fish waste into relatively harmless substances. Sponge filters of some sort work well and a pond or tank should have a bare bottom for ease of cleaning. This bare bottom allows one to see any uneaten food that has sunk to the bottom. It can then be easily removed. Uneaten food is always a fish killer if in great enough quantity.

Water changes will need to be made in any indoor pond situation. The preferred amount of water to change at any one time is 20% of the volume of the pond. Of course, the make-up (replacement) water will need to have the chlorine removed. The more the fish are fed, the more water changes will be needed to remove accumulating substances. Contaminates still build up in the water column even with the right filters. If the indoor pond is kept at 50°F. or less, then feeding is not necessary. With this cooler situation, water changes once every month or two will be all that is necessary. Warmer water means a water change of at least 20% every two to three weeks.

The process of moving the fish back into the outside pond in the spring is somewhat simpler than moving them inside. It is the time to be thinking of moving hardy fish back to their outdoor pond when the spring thaw occurs. Even if it freezes at night the light ice that is formed will be gone in a few days. This light ice will not be harmful to the fish. So, move them early in the spring.

However, with most tropical fish wait to move them outside when water temperatures are above 65 to 70°F. As in the fall, take your time acclimating the fish, especially if there is a large temperature difference. Again, acclimate them to the water chemistry as well as water temperature.

In areas where winters can get really cold, bringing fish inside when the weather turns nasty means they will never know how brutal winters can be. Keeping the indoor pond cool, making adequate water changes, and acclimating them correctly are all important for the healthy survival of your fish.

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36 thoughts on “Overwintering Pond Fish Indoors”

  1. I am moving to a house with no pond and I would like to take my fish with me. How would I go about moving my pond goldfish to a tank?

    1. The first step is to decide the kind of tank and size. If this is a permanent set-up for your fish, you may consider an aquarium so you can enjoy them even more. The biggest hurdle with an aquarium is the cost and placing it. Livestock stock tanks work very well and if you look at the pictures associated with this article — that is what those are. They come is all sizes so how big do you go? Refer to my article to give you some pointers in deciding.

      I go step by step in moving your fish from the pond into your tank in my article. I also go over taking care of them in the article. Good luck and enjoy your fish.

      Also, it is a good idea to build a pond at your new house as soon as you can. Sorry — I couldn’t help but say the obvious.

  2. I am getting prepared to bring my Shubunkin and Sarasa goldfish in for the winter. We tried to leave last year’s fish outdoors overwinter, but they all perished. I have 5 adults (approx 4-5″ each) and 20 babies (1″ at largest). I am going to keep them in a 50 gallon “indoor pond” with a low temperature – water trough, 50 degree water. What do you recommend I use for the aeration/filtration/circulation (products, sizes etc)?


  3. Avatar photo

    From Jamie Beyer:
    A 50 gallon container that you have is small for the number and size of fish that you have. It should work as long as you make water changes during the winter. The 50 degree temperature is good. The fish can easily tolerate even lower temps to say 40 to 45 degrees, without any harm to the fish. I would not feed the fish as long as the temps are 50 degrees or lower. Please refer to my article on “Overwintering Pond Fish Indoors” . It addresses all the issues of moving fish inside.

    You must provide aeration along with filtration. I use sponge filters. Do not have any gravel on the bottom of the tank so that you can easily clean any sediment out of the tank. I normally remove sediment during my once/month water changes. Remove at least 20 to 30 % of the volume and replace.

    Good luck and have fun water gardening. Jamie

    1. Thank you Jamie. Thinking I will look into getting a 75 gallon tub. Is there a special type of thermometer I need to get for keeping water cooler, rather than warmer?

      Cheers, Jess

  4. I have an outdoor pond that’s something like 2 feet deep with about 5 adult goldfish and their 15 or so spawn goldfish :). We live in the Chicago suburbs so obviously there’s a cold winter apporaching. I don’t have the capability to get electric to the pond currently so am exploring alternate options. Is it possible they could survive in the pond over the winter? Are there non-electric methods of keeping an outdoor pond safe? I’m hesitant to bring them indoors since there’s not a great space to keep a tank or pond liner, curious to get your advice on my options (short of depositing them all into a lake somewhere or taking them back to the store where I got them in the first place).

    1. Avatar photo

      From Jamie:
      This is a good question and one that deserves an answer.

      I used to overwinter my ponds without any kind of heater nor aerator (35 plus years ago) but my fish population was very low and I made sure there was not any organic matter in the pond. Organic matter is anything that was alive and could decompose in the pond. Leaves and dead critters immediately come to mind when speaking of organic matter.

      The key to this technique is not many fish in an in-ground pond that has some volume to it. Since your pond is two feet deep there is not much volume to work with. You did not give me the size of the pond but I am assuming that it is relatively small. The pond ice can easily get to 15 inches thick in urban ponds. So in a 24 inch deep pond the fish potentially only have 9 inches of ice free water that has to hold enough oxygen to last the entire winter. In your situation I would say that it is too much of a gamble. Now, if your pond was 3 feet deep then it would be a safer bet but it is still a gamble. That is why I always recommend an aerator.

      If I were you, I would look into a solar powered aerator or a wind powered aerator. You still would need to remove the organic matter but I think that this would work for you. I have never used these devices but I think they would work okay. You still run the risk of them not being enough aeration in certain situations but at least you have tried.

      By all means do not release your fish into our lakes and rivers. Taking them back to the fish store is a good idea if you do not want to go through the above effort.

      Good luck and have fun water gardening. Jamie

  5. According to your article, when making water changes in an indoor aquarium it has to be 20% of the volume of the aquarium or indoor pond.
    The water replacement will need to have the chlorine removed apparently. How do you remove the chlorinated water?

    Thank you for a response.
    Much appreciated.

    1. Avatar photo

      Whenever chlorinated water is used where fish and other aquatic animals occupy, then it has to be removed. There are many chlorine and chlorimine removers on the market. Use them according to their directions, of course. I use a chlorine remover that I add as I add water to a tank or pond. Simple and easy.

      Have fun water gardening. Jamie

  6. I inherited a fish pond when we bought our new house. Our pond is small and shallow so we brought our goldfish in for the winter. From reading your blog and comments, you seem to recommend treatment as close to what they’d get outside while keeping them indoors for the winter, but will they be ok if we set up the aquarium as if they’d live there forever (rocks and plants, light, feeding) or do they need to be in “winter mode”, cold and food free. My basement is finished and my garage is unheated, so they’ll either be too over or too under the 45 degrees…


    1. Avatar photo

      From Jamie Beyer:
      One of the main reasons I recommend keeping the fish cool for the inside treatment is so that you do not have to feed them. This means you do not need to make as many water changes. Obviously the fish do not create as much waste when they are not fed. You can also have more fish/gallon in cooler water due to the above reason.

      The fish will do fabulous in an aquarium situation in warmer water. Give them as much volume as you can. Going from a pond to an aquarium is normally a huge volume difference. This makes me nervous — but as long as you do not have a lot of fish and they are relatively small then this option will work fine. Make sure that you feed them and do your water changes. The aquarium is a fun option and one that people do to enjoy their fish even during the winter.

      The unheated garage sounds like a great option as well as long as the water does not freeze inside the garage. If the water is around 38 to 40 degrees F that will be okay. The 100 gallon stock tanks mentioned will house the fish easily and they are relatively cheap for the gallons that they are.

      Good luck and have fun water gardening. Jamie

      1. Thanks for the help! Our aquarium is ~55 gals and I forgot to mention we only have 9 goldfish that are all under 3 inches long, so they have tons of room. Any liquids left in the garage overnight get slushy, so based on that, I think we’ll keep em in the basement. You should see how active they’ve been in the tank since it’s warmed up to room temperature. I’ll be sure to change the water and now I’ll give em some food to keep em active.

  7. I have a small blue kiddie pool on my patio with two potted plants, a small fountain pump that sits in the middle to flow water, and about ten basic goldfish. I’ve had this set up for a few months and no issues or deaths. I live in central Texas and we just had a hard freeze. I put an aquarium submersible heater in the ‘pond’ with a powerhead to circulate water over heater. I set the heater to 72*F. Fish were doing fine. Then yesterday I got home and went to feed them and they were all dead?! I checked the water temperature and it was about 67*F. Is my setup wrong? Did I make the water too hot? I did notice green algae on the thermometer – is the water too polluted? I have never changed the water, my dogs drink out of it, and as it evaporates I just keep filling it.

  8. Ashley

    From your description it appears that the fish did not die from being too cold or too hot. It is interesting (and sad) that they all died at about the same time. Your question about the water being to polluted is a possible answer especially if you do not do any water changes. It is also possible that the aquarium heater had a short in it and the fish were electrocuted. If there is a GFI on the circuit then this would not have occurred as easily. If the heater was old and the cord frayed that was in the water — then this is possible.

    In the future, you would not have had to worry about heating the water with red comet goldfish. On my outside tubs where I raise thousands of calico shubunkin goldfish they routinely will get a layer of ice on them before I get them moved inside for winter. They are adapted to cold water.

    Please do water changes as I have mentioned in my article. This is a critical part of keeping fish in containers and aquariums.

    Good luck Jamie

  9. Penny Ludwig-Sarx

    Hi, we live in Kitchener, Ontario, Canada. We have had our pond for over 10 years and have kept them in the pond every winter. However, we have had good years of losing none and other years where we have lost many. This year we have decided to bring them indoor in our attached garage. I have read the above article and have some questions. Do we need to be concerned with light? Since the pond freezes over with ice and snow (other than an open hole from the aerator) it would be completely dark for them. Do we need to provide that kind of environment (darkness)? Will noise stress them. Examples: Like the snow blower coming in and out of the garage when we use it? Or a car that will come in and out of garage when used. We plan to use a 150 gallon container (very similar to the one above in your article). Should this container be put on a platform (ie sheet of styrofoam) to keep off the cement floor? Look forward to your response. Thank you!

    1. Avatar photo

      Answer from Jamie Beyer:
      Hi Penny
      All good questions.

      In all reality, there is some light that penetrates the ice and snow but not much. If there is not a window in the garage then I would provide a small light on a timer to mimic a light cycle. Otherwise, when you open the garage door and bright light comes in — it could spook the fish.

      The noises that you mentioned should not be a problem as long as long as the equipment approaches the container slowly. Sudden noises could be a problem as far as spooking the fish.

      Having the container on a platform is not necessary. Actually, I would prefer that you place it on the concrete floor. The concrete will be a uniform temperature relative to the ambient temperature. The concrete temp will assist in keeping the container water a more constant temperature.

      Have fun water gardening. Jamie

  10. We made a pond for a little turtle I found last year (we build and maintain in ground pools) when we were changing a pool liner. The pond is 12′ long and at the widest 7′ wide and the deepest area is 2′ deep. The pond gradually goes up to 9 inches deep with then a small area that’s almost dry so the little guy could sun bathe. We put him in the pond and went to dinner and by the time we got home he had escaped over 18 inches of concrete blocks!!! So we converted it to a koi and goldfish pond.
    We live in upstate SC so winter isn’t usually bad but this winter has been tough! Thankfully all of my 4 koi and 2 goldfish babies have survived. We did have to add a homemade filter with rock and sand that then goes to another much larger filter we bought that has sponges and biological filters and the light. We are planning on moving the babies inside so we can completely redo the pond and make it deeper (3 to 3 1/2 feet deep all the way around) and make the sides straight down to protect from wildlife and to add large rocks on the sides that will give the babies hiding spots. The largest koi is about 12 inches long and she’s a chunky girl ☺️. The goldfish are the smallest and are about 6 inches long and they are little chunky babies too. They are all very healthy and absolute baby dolls (yes I have grown EXTREMELY attached in the 9 months we have had them).
    What I need to know is if I bring them in for about a month (shouldn’t take that long to finish the pond but just over estimating) in a 64 gallon tote with 50-75% of their pond water and their biological and sponge filter and keeping the water under 50° like it is now outside, what else do I need to set up the temp housing? Do they need rocks? Do I need to put plants in? (There aren’t any plants in the pond because I haven’t been able to find any that will survive the winter)
    Also since the water is so low I haven’t fed since the beginning of November so I will try to keep the water below 50° but I’m not sure how since they will be inside (it’s the only safe place to put the tote)

    1. Avatar photo

      From Jamie Beyer:
      Hi Codi

      I am impressed by your dedication to keeping your fish healthy. They do become pets that are very dependent on us.

      Keeping the water cool will help with how easily it is to keep the water healthier for the fish. The warmer the water the higher the metabolism of the fish and the more they excrete waste. Regular water changes are a must to keep the nutrient load down. In my indoor tubs where I have perhaps 50 – 5 inch Calico Shubunkins at 75 degrees F, I make huge water changes every 6 to 7 days. I am feeding the fish and could possibly feed less. Then, I would not have to make as many water changes. However, at that temperature the fish are still growing and metabolizing calories so they do need to be fed.

      So, if you can keep your fish at 50 degrees and fewer fish in a 100 gallon tub then the water change schedule would be less. You would not need to feed as much, of course — if at all, at that temperature. You could only feed once/week just a small amount. I would still do a water change like I say in my article.

      Putting plants into an indoor tub/container is a possibility. However, you would then need fairly intense artificial light or be in a greenhouse with your planted tubs. If I could do this practically and economically, I would. The plants do absorb a lot of nutrients out of the water and are extremely beneficial when you can add them. Without plants your water changes will keep the nutrient load buildup to a minimum. Of course, you can add biological filtration which will also reduce nutrient load. You will still need to do water changes even with plants and bio filters but maybe not as often. This depends on types of filters and plants. It will be guessing game so I err on the side of too many water changes.

      As far as putting rocks/gravel into an indoor tub — I would not advise this at all. It is easier to make water changes and keep the tub free of waste by not having to negotiate rocks on the bottom. It is much better to add a bio filter than to add rocks. Rocks also take up volume and I would prefer to have the extra volume to dilute nutrient buildup.

      Have fun water gardening. Jamie

  11. Hi I had 9 beautiful pond fish, I believe goldfish with long fins, which were so pretty. Had them in an aquarium tank with filter going and I changed water as needed. They were doing very well 6 months in, then I noticed one day not as active, then they were all dead the next day. What happened? Feeling sad, where did I go wrong?

  12. Hi Diane

    I am feeling sad as well but will attempt to make a couple suggestions to help you figure out what happened.

    As soon as a fish keeper notices fish not being as active and/or not eating much food as during previous day’s feedings then action needs to be taken THAT DAY. ( I am assuming that the fish are not gasping at the surface for air.)This is true for pond fish as well as indoor tubs/aquariums. You first need to determine if the fish have been spooked for some reason. New fish added or someone tapped on the tank or whatever can be a cause of fish not acting normal.

    If not spooked then it is very possible that the water quality degraded to a point where it is potentially lethal. This can literally happen overnight in small ponds or containers. The potential remedy is to immediately make a water change that day you see them not as active. If the fish are not as active after the water change the next day — make another water change. Water changes of at least 20 percent is critical. I often do a much larger water change.

    Look for uneaten food on the bottom of the aquarium/pond while doing a water change. If so, then suck this out also. Uneaten food is a sure sign of something not being right. Feeding too much where the uneaten food rots on the bottom causes the water quality to degrade fast.

    Most diseases take longer to kill fish. Notice I said “most” but normally one or two fish will become listless or not eating when a disease starts — not the entire fish population all at once.

    If it is a disease, water changes are almost always good for the fish even then. You can easily test the water for ammonia or nitrite levels to go a step further.

    Finally, the larger the aquarium/tub/pond and/or the less the number and size of fish then the potential problem of water quality degrading fast becomes less of a potential danger.

    So, what water quality parameters are degrading? Who knows for sure — could be a pH crash, could be ammonia or it could be nitrite to name some. The water changes will dilute all of them.

    Remember the “solution to water pollution is dilution”. You will be diluting the problem polluting factor thereby saving the fish.

    Good luck. Jamie

  13. I live in Minnesota (twin cities) and have two ponds with Koi. In the past I have wintered my fish indoors, but last year in the middle of the winter had a filter go bad and lost all my fish. As the fish get bigger (my last set were 5 years old) my indoor tank no longer seems suitable for them. So I have decided to keep them in a 100 gallon stock tank in an attached garage. I have had the water in the tank with 2 small in-tank filters run via a bubbler.
    I have done a lot of reading and I have a couple of questions as this is my first time doing this in a garage, and this is Minnesota.
    * I bought a controller that I can accurately measure the temperature of the water, currently 48 (October 13th) and you can turn on/off a heater(s) at a on temp and off temp. I am thinking turning it on at 38 and off at 40. Does this seem like a good temperature range to maintain?

    * I am also wondering if a 300 watt eheim heater would work in this colder temperature but rely on the controller to turn on/off to keep it in this range. I could purchase a bucket heater but think the eheim would work better and easier to maintain the 38 to 40 degree water temperature. Note the display for this heater can be easily seen when I enter my garage so if needed I could add one more heater to help keep up (I have a few spares – I also have a saltwater reef tank so I have a lot of fish stuff). Also I am going to cover the stock tank with form sheet of insolation, to help maintain the temperature if needed.

    * One other question is I am thinking of bringing them in late October/early Nov as the temperature in the Stock Tank will be consistently below 50. Does this seem right? Currently it is measuring at 48 but has been unusually cold for the last week, and I have just got the controller up and running.

    Thanks in Advance!


  14. Good questions Keith.

    I would prefer to keep the temperature of your stock tank between 40 and 45. If you keep them in the 30 degree range you might as well keep them outside. I would use an aquarium heater like the Eheim that is thermostatically controlled. A 300 watt heater should be enough but it depends on how cold the garage is. If it is an attached garage then the ambient temperature should be higher.

    You did not say how many fish nor size of fish. Since they are Koi, I would make sure that you give them enough space. The colder water of 40 degrees is good since you will not be feeding and they will not be excreting much.

    I do like the insulation on top but make sure that your aerator is working ALL the time. The insulation will not only keep the tank warmer but it will also keep oxygen from the surface UNLESS you are pumping air under it. I have experienced fish suffocating in a situation like this when the power went out. Some of this is dependent on how much air space you have above the water surface and the insulation. The more air space then the longer you can go without air being pumped into the tank. Maximum time without air being pumped under the insulation is dependent on the number/size of fish. But any longer than a few hours is gambling.

    I would also place a sheet of insulation on the floor under the tank. The transfer of cold from the concrete floor can be substantial. In the past, I have also sprayed on insulation to sides of tanks but this is more costly.

    You can bring them in anytime from now on. I am one to wait until right before ice forms. I would also transfer pond water into your indoor tank. At least one half of the volume should be pond water. It is a large shock for the fish to go into this situation so why not minimize this shock by putting them into water they are acclimated to?

    Remember to do a few water changes during the winter.

    Have fun water gardening. Jamie

    1. Keith again – thanks for the help last year!

      This set up that I got recommendations from you worked great and I am happy to say that all my Koi made it great through the winter. Having both done wintering indoors and now in an unheated garage I much prefer the garage solution as it is easiest, cheapest, and safest for the Koi by far. Also by using the unheated garage method, the Koi stay in the pond much longer – from end of April to November – which is also a benefit.

      I have 6 Koi that are now about 10 inches long, I am thinking that a 100 gallon stock tank with 6 – 8 koi seems like the right number.

      As I prepare for this winter, I wanted to provide some additional details on my Koi wintering set up in case it helps others.

      To the above configuration I additionally added an under-water led light on a timer and set to an aqua blue setting to simulate day/night, thinking this was a good quality of life enhancement and allow for me to check on the water quality about every 3 weeks. On that note the quality was crystal clean all winter, so I did no water changes. This year I will test the nitrogen levels about every 3 weeks to confirm this.

      Here is the process I used:

      In late September: I set up and filled the stock tank up to about 3/4 full of water in the garage and added conditioner, and put wood boards under the tank and covered surrounded it with old patio furniture cushions to help keep the heat in/cost down. I start the in-tank filters (with a two tube out pump), which have aerators to run the nitrogen cycle and stock tank conditioned. And set up the heater and external temperature controller and start watching the temperature. The controller has large red digital number so easy to monitor each day as they are easily seen/monitored daily.

      In early November – when the temperate of the stock tank gets to or below 49 F: I add the additional 1/4 of water from my pond and move my fish from the pond to the stock tank. Cover the tank with Styrofoam insulation to keep heat and the Koi in.

      I watched temperature all winter and have the controller set at 39 to turn the heater on and turn off at 41 . Again, I am using a temperature controller (BTC211) hooked to a 300 W Eheim heater. This worked great, in the dead of winter the temperature never went below 39 (or above 41) and I only needed one heater. Also could hear the aerators so made sure those were also always working.

      After winter I cleaned out my pond in the first nice weekend in the first week of April, when I noticed the garage temperature starting to slowly rise. I waited a couple of weeks for the garage temperature to get to about 49, and the pond to cycle. Also monitored the outside temperature of my pond and made sure it was about 49 too. Then moved the fish back outdoors on April 24.

      Hope this helps others, again I really like this set up and this being the second year I also like that I just can reuse everything, nothing to buy.

  15. What if I have a garage which might average under 50 degrees, but may periodically rise to 69 degrees for hours at a time? Do I need to feed them during these periods? Thanks

  16. Good question. I would check the water temp — if it rises above 50 degrees then consider feeding some. The fish, a lot of times, will tell you — “feed me” — by begging at the surface. If they are right there then I would feed some but not too much. The fish can actually go a long time without feeding even at 55 degrees or 60. Have fun water gardening. Jamie

  17. Hi! I know you wrote this a bit ago so not sure if you are still responding. It’s a wonderful article and gave me some excellent information, but some of my questions are actually opposite of just over wintering inside. I have a 50 gallon, above ground, patio pond with a comet, fancy tail, and what we believe may be a Sunny! (Was using a tiny feeder to help cycle the original set up and he has grown up and done great). They are each maybe 6”-7”.

    I want to bring them in over the winter since I am in PA and it is an above ground pond. I will be moving them into a new, 110 gallon set up for fall and winter. The article is more about going from bigger to smaller, but mine is opposite. Is it ok to set up the pond in the main living area and keep them as you would any other aquarium fish, just in a pond? I would not think they need to be 45-50 degrees unless I am looking to almost hibernate them correct?

    Also my water volume will be far different so I won’t be able to add more then about a 35% of old water to fill the new 110. I am hesitant to use the older water because of possible pests (larvae and such), does this tend to be a problem in the house? The pond outside is planted and again, afraid to transfer plants for the same reason, but they have heavy cover of water hyacinths presently and I am afraid without them they will be stressed. Will they adjust to not having them?

    I want to make the stress on the fish as minimal as possible. They started out quite tiny a few months ago and have done excellent outside and I think will love the larger space. The have an internal, canister filter rated for up to 150 gallons which I plan on moving to the new set up, as well as a small, “fountain” for added oxygen. Is there anything I need to do differently since I plan on keeping them where I can watch them instead of just over wintering? Thank you so much! And thank you for all of this awesome information!

  18. Hi Liz

    In the move, I would worry more about the temperature and water chemistry change than anything else. More fish are killed by transferring fish into different bodies of water. The most important thing is to acclimate them slowly to the new water they are going into. Do exactly the same thing as my article says. I would not worry about transferring pests/diseases. It the water they are in now has them — they would already be compromised. Remember the warmer water will require more feeding and then this requires more water changes. Outside natural rains provide water changes as well as UV light (sunlight) does wonders for fish health. Since you do not have this inside, then do your water changes.

    The fish will adjust to not having the plants. However, those plants provided a hugh phytofilter (plant filter). Since you will not have them then the water changes are even more important. The water hyacinths will normally die inside unless you provide intense artificial light. The canister filter is a good idea but clean it often.

    Water changes are vital for keeping your fish indoors at warmer temps. Good luck and have fun water gardening. Jamie

  19. Laurie Quenneville

    We have an outdoor pond and live in Ottawa Ont. Canada. We overwintered our goldfish in the pond last year and we lost most of them. We have a heater and aerator that we put in the deep end which is only 2 1/2 – 3 feet deep. There was always open waters throughout the winter but I am not sure if the pond was just not deep enough or a predator got them. We have a few koi, 6 -7 ” and 3″ goldfish and a bunch of babies. We now want to put them in a stock tub 4 x 6 x 2′ deep, which we can keep open with a heater and aerator as well. The question is to put the stock tub on the deck or in our heated garage? Also by the ? and answers I don’t need to put anything in the pond, ie cave.

  20. Hi Laurie

    It is unfortunate that you lost a lot of your fish last winter by leaving them outside in the pond. Given the situation that you presented by placing your fish in a stock tank, I would put it into the heated garage. Not sure of the size of the stock tank but I doubt whether it is larger than 100 gallons. However, I would not go smaller than 100 gallons with the number and size of the fish you are placing in it. More volume is always better when it comes to keeping fish.

    It would be very expensive to run the heater while the stock tank is on the deck. It would take a high wattage heater to heat it enough – depends on the severity of your winter.

    Your heated garage only needs to warm enough to keep the water from freezing. I do prefer a colder situation for your fish like my article says.

    Good luck and have fun with water. Jamie

  21. I hope you can respond: I live in southern Washington state I have two full size goldfish left, cats and raccoons ate my others. I have to completely redo my pond so no animals can get to the fish. I have a 25 gallon fish tank in my house, I was going to fill it with pond water but the water temp outside is 52degrees. What more needs to happen before I bring my good fish in?? They have lived in the pond for 4 straight years. From warm water summer to ice cold freezing on top winter.
    Thanks -Don

    1. Hi Don

      You mentioned that the goldfish are “full size”. I am assuming that they are a foot long or so. If that is the case that 25 gallon tank is a bit small. It will work but you will need to make water changes more often in that small of a tank. I would prefer a larger tank if it were me. A 100 gallon tank or larger would work. I like larger volumes of water since smaller volumes can more easily be polluted by fish waste or even overfeeding.

      Like I say in my article the key to moving them in is a slow acclimation from the pond to the indoor tank. I would bring in 20 gallons of pond water and place the fish immediately in that water. That amount may only fill the bottom of the indoor tank. It may take a bit more than 20 gallons of pond water depending on the size of tank that you use. you want the fish to be able to swim in the tank. If you end up using that 25 gallon tank then bring in 15 gallons of pond water. Make sure you are aerating the water. Then add 10 % more of the fill water that is dechlorinated. Wait an hour or two and then add 10 to 20% more fill water. Wait another hour or two and then add 20 to 40% more fill water. Keep increasing the amount of percent until the tank is filled. The fill water can be 70 degrees but as long as you are acclimating this slow that is okay.

      Remember you are acclimating to not only temperature but also to the water chemistry (pH, hardness, etc).

      Have fun water gardening. Jamie

  22. Thanks for the advice last Fall. We are in Ottawa, Ontario. Our koi and goldfish were brought into our heated garage over the winter. Even though the garage was around 65 F the stock tank got to around 70F – 74F. It is 6′ x 4′ and 2 feet deep, there is about 25 – 30 fish in there. We have a really good filter and have an aerator going all the time. We are feeding them twice a day (all winter)and did/doing water changes every 3 weeks. So far they are doing well. We have noticed just lately that some of the fish that we were not able to catch, are waking up in the pond now which is about 50F. They were more fortunate than the frogs, I took out alot of dead frogs this year. :-(. Getting to my question, which is, when can I move the koi and goldfish that are in the stock tub to the outdoor pond. The pond is pretty clean as I have removed all the dead frogs I could find and most of the leaves. We started the ion gen, so the water is clear. I worry about the temp difference between the pond and tub. I know they can tolerate the cold but I want to minimize the stress in moving them. Should I wait until the temps are closer,i.e. 10 degree difference? Thank you in advance, I really enjoy your site and reading everyone’s question and your answers.

  23. Thanks, Laurie, for your question. Your indoor pond is perfect in size, and it sounds like you’re doing everything right with aeration/filtration and the water changes. I am impressed and I think your fish are as well. 🙂

    You are correct in assuming that a large difference in temp can be fatal but so can water chemistry (like pH or hardness) be fatal. So it is important to acclimate (look up in the article on how to do this) anyway — even if the temp is close.

    This time of the year our fish really enjoy spring as much as we do so get those guys outside. Sorry to hear about your frogs — I have been thinking of techniques to overwinter them in your pond. Maybe a future article. Good luck and good luck water gardening. Jamie

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