Track my Order  |  My Account  |  0 item(s)    View Cart

Overwintering Pond Fish Indoors

by Jamie Beyer

Overwinter koi fish outside

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, 1000 gallons that are at least 2.5 feet deep can successfully overwinter hardy fish without supplemental heat during these severe conditions. Please refer to my Sept/Oct 2009 Pond Trade article (, "Overwintering Koi Under Thick Ice." 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.

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."

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."

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.

- Pond Trade Magazine January/February 2013