Overwintering Pond Fish Indoors

100-gallon containers make excellent indoor ponds for overwintering hardy goldfish. This size can easily contain 50 to 75 – 3˝ Calico Shubunkin goldfish if kept at 50°F. Aeration is provided along with large sponge filters. In this situation, the containers are not covered because the fish are small and will not normally jump. If larger fish were kept in this situation then the container should be covered with a jump-proof type of cover.
100-gallon containers make excellent indoor ponds for overwintering hardy goldfish. This size can easily contain 50 to 75 – 3˝ Calico Shubunkin goldfish if kept at 50°F. Aeration is provided along with large sponge filters. In this situation, the containers are not covered because the fish are small and will not normally jump. If larger fish were kept in this situation then the container should be covered with a jump-proof type of cover.

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.

 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.”

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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.

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.

23 Responses to Overwintering Pond Fish Indoors

  1. Lily April 16, 2015 at 12:37 PM #

    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?

    • Jamie Beyer April 25, 2015 at 11:49 AM #

      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. Jess September 21, 2016 at 2:43 PM #

    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)?

    Cheers,
    Jess

  3. Lora Lee Gelles September 23, 2016 at 6:44 AM #

    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

    • Jess Urquhart September 30, 2016 at 2:28 PM #

      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. Tim September 30, 2016 at 11:28 AM #

    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).

    • Lora Lee Gelles November 27, 2016 at 9:22 PM #

      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. tim b wilson November 1, 2016 at 7:25 AM #

    I’m over wintering gold fish that are 1 to 2 inches at 45 degress F. These little guys are not to be fed?

    • Lora Lee Gelles November 4, 2016 at 9:14 PM #

      Once the water temperatures go below 50°F, stop feeding your fish until the water warms up in the spring.

  6. Fabian Rodighiero November 25, 2016 at 10:13 PM #

    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.

    • Lora Lee Gelles November 27, 2016 at 9:19 PM #

      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

  7. Jeff December 12, 2016 at 9:20 PM #

    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…

    Thanks!

    • Lora Lee Gelles December 14, 2016 at 12:46 PM #

      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

      • Jeff December 14, 2016 at 10:07 PM #

        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.

  8. William ORLOVE August 17, 2017 at 11:58 AM #

    What about lighting? Do you have to mimic daylight, turning lights on and off?

  9. Ashley Adami January 3, 2018 at 3:32 PM #

    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.

  10. Jamie Beyer January 4, 2018 at 12:23 PM #

    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

  11. Penny Ludwig-Sarx October 1, 2018 at 6:39 PM #

    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!

    • Lora Lee Gelles October 2, 2018 at 10:03 AM #

      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

  12. Codi February 22, 2019 at 8:01 PM #

    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)

    • Lora Lee Gelles February 23, 2019 at 2:53 PM #

      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

  13. Diane March 28, 2019 at 3:57 AM #

    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?

  14. Jamie Beyer March 31, 2019 at 1:27 PM #

    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

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