Bringing Pond Fish Indoors in November: When It's Worth It and How to Do It

The leaves are down, the morning water in the pond is cold enough to make your hand ache, and the goldfish have gone slow and quiet near the bottom. You start wondering whether they'll be all right out there or whether they should spend the winter in the basement. For hardy goldfish and koi in a pond that suits them, the answer is usually that they can stay out. If your pond can't keep them safe, or the fish are fancy or tropical, November is late but still workable for bringing them in. Here's how to do it without losing any.

Do you need to move them?

Often you don't. Hardy goldfish and koi are built for cold water. At about 40–50°F, koi stop eating and settle near the bottom in a dormant state, with a body temperature that matches the water around them. A pond that's deep enough not to freeze solid, with an opening in the ice for gas exchange, gets a lot of hardy fish through winter. Outdoor overwintering doesn't always go to plan, though. How it turns out changes from year to year with how harsh the winter is, how much leaf litter and waste has built up, and how healthy the fish were going in.

Bring them in if:

• The pond is shallow or you can't keep a hole open. Without that, the fish are taking their chances.
• They're tropical. Tropical fish need to come in before the water drops below 65°F, and in most places that date passed weeks ago.
• They're fancy goldfish. Egg-shaped bodies and trilobed tails mean less cold tolerance, so bring them in for a hard winter.
• You want to watch them. That's a fine reason too.

Hardy goldfish can come in any time before the pond freezes over for good, so November isn't too late for them.

If you've ended up with more fish than you can keep inside, don't release the extras into a creek or lake. Michigan State University Extension says never to do that. Take them back to the store, trade them with another keeper, or donate them to a school, a public aquarium or an aquatic business.

Pick the tank and set it up cold

Bigger is better, and it's cheaper than you'd guess. Pond Trade Magazine recommends stock tanks of at least 100 gallons. It treats a 50-gallon container as tight even for five 4–5 inch goldfish, and only works if you change water often. A 25-gallon tank is too small for full-size goldfish. A more recent Pond Trade article gives a rule of thumb of about 30–40 gallons for each 4–6 inch fish.

The cheap containers are the big ones. Stock tanks run from 150 to 750 gallons, and an IBC tote usually holds 275. Either one costs about $150–$275, while a 240-gallon glass aquarium runs upwards of $3,000. Unless you want a winter display, the tote wins.

Set it up like this:

• Filtration: sponge filters or another biological filter.
• Aeration and circulation: Pond Trade calls them "critical." Run an air pump even if the filter moves water.
• Bottom: leave it bare so you can siphon sediment out easily.
• Lid: use heavy welded wire, hardware cloth, Plexiglas or glass, and weigh it down. Flexible plastic and tarps don't hold. Koi keepers on forums say the same thing about lids: "they WILL jump."

Keep the water cold. A garage or basement that holds around 40–50°F makes everything easier. Cold systems can hold more fish, and a system running near 70°F needs more filtration. A warm living room means a faster metabolism, more waste and more work for you. Heating a large tank isn't cheap either. One koi keeper on a forum reported spending about $1,600 every two months on several heaters before giving up on heat altogether.

Cycle and move the fish

The filter needs a working colony of bacteria before any fish go in. Cycling first, using a dechlorinator and testing the water are all on Pond Trade's list of basics. Experienced koi keepers allow four weeks or more for cycling. One suggests feeding the empty system ammonium chloride at about 1 teaspoon per 400 gallons while the bacteria get established. With Thanksgiving falling on November 26 this year, a tank set up in mid-October would be about ready around then, so start now. Keep testing until ammonia and nitrite both read zero.

On moving day, bring the pond with you. Moving 20–50% of the outdoor pond water into the indoor tank cuts down on acclimation stress.

Then let the temperatures meet slowly. Pond Trade's method is:

1. Net the fish into a container of their own pond water.
2. Add indoor tank water equal to about 10% of the volume and wait 15 minutes.
3. Add about 20% more and wait another 15 minutes.
4. Keep going in steps until the container is mostly tank water, then move the fish across.

If the pond is at 45°F and the basement tank is at 60°F, stretch those waits from minutes to hours. Cover the bucket the whole time. Stressed koi jump, and the lid matters most during the move.

Feeding, temperature and water changes

Keep hardy fish as cool as you can without letting the water freeze, ideally 40–50°F. That cold is what makes indoor overwintering easy.

Feeding. At 50°F or below, they don't need food. Their metabolism has slowed so far that food mostly just rots. Outdoors, pond keepers stop feeding completely once the water reaches 55°F and switch to wheat-germ food in early fall as the water cools. Wheat-germ food is the usual pick for the cool range between about 40 and 50°F, if the fish are moving around and you feed at all. Above 50°F, feed regularly but take out uneaten food right away. Pond Trade calls overfeeding "a fish killer," and indoors that's even more true than in the pond.

Water changes. Change at least 20% of the volume each time, and always dechlorinate the new water.

• At 50°F or below: every 1–2 months.
• Above 50°F: every 2–3 weeks at the least.

Testing. Test ammonia, nitrite and nitrate. Any elevated ammonia or nitrite is deadly, so act right away with a water change and less food. Nitrate above about 80–100 ppm may not kill koi or goldfish, but it means a water change is due.

Mistakes and when they go back out

Most indoor overwintering losses come from a short list of mistakes:

• A tank that's too small for the number and size of fish.
• Skipping acclimation, or dropping fish straight into water at a very different temperature.
• Overfeeding, especially in cold water.
• No lid, or a flimsy one.
• Warm living-room water with the same filtration you'd use for a cold basement, which leads to ammonia and nitrite spikes.

In spring, reverse the move. Wait until the pond is thawed and its temperature is close to the tank's, then use the same slow, stepped acclimation to get the fish back outside.

The lid lesson is one I learned with my River Tank. I swapped the light kit for a full screen lid because crabs and some reptiles get out of almost anything. Koi are no different. A cold, well-filtered tank with a heavy lid will get them through until spring.

Sources (6)

• POND Trade Magazine: Overwintering Pond Fish Indoors (Jamie Beyer)
• POND Trade Magazine: Creative Indoor Setups for Wintering Koi and Goldfish (Andrew Lohman, 2025)
• Northeastern University News (Jan 20, 2021)
• Michigan State University Extension: Avoid spreading aquatic invasive species when winterizing your water garden
• KoiPhen forum thread (Aug 2015)
• Koi Villa via Patch: Closing Down Your Koi Ponds and Water Gardens (2012)

Photo by zoosnow on Pixabay.