How to Overwinter Fish Indoors
If you have a fish-filled pond, there are two options for winterization: overwintering your fish in the pond or moving them indoors. Ponds that are at risk of freezing solid, like small or above ground ponds, cannot overwinter fish. Instead, you can overwinter fish indoors in a separate tank or aquarium. Overwintering fish indoors can help protect them from harsh winters and the risk of high stress or sickness. Learn more about overwintering fish indoors.
Prep Work
Do you need to move your fish indoors or will they be okay in your pond? If you live somewhere cold enough that your pond is at risk of freezing solid, you'll need to move your fish indoors. Some surface ice is ok, but only if you can keep a hole in the ice. Learn more about if you can let your pond freeze over.
It's essential to make sure your pond is as clean as possible through fall, up until you remove your fish for winter. Remove debris and use sludge remover, if needed, to keep your water clear. Ideally, you'll give the indoor tank a month to settle before adding the fish so beneficial bacteria can colonize and cycle. The fish's transition is easier if you use water from the pond in their indoor container.
Setting Up An Indoor Tank
It's essential to wait until the indoor tank is ready before you relocate your fish. The process can be very stressful for fish. When setting up the indoor tank, you should use at least 20 percent of your pond water, but ideally around 50 percent, to make the transition easier.
If you have room, you can add aquatic plants to the tank to help filter out the fish waste. You will need to use an aerator and, ideally, a filtration system like biological filtration to keep the water oxygenated and clean.
What Kind of Container to Use?
The indoor tank should be big enough that the koi can turn around completely. Overcrowding can lead to water quality issues. The container needs to be fish-safe and clean. You can use a preformed pond liner, stock tank, or aquarium. Koi are known for being jumpers. The tank must have a cover that the koi can not move. You can use pond netting or wire.
Temporary Holding Container
Fill a temporary container big enough to hold all of your fish (alternatively, if you have fewer fish, you can use clear plastic bags) with pond water, but leave room for about 50 percent more water to be added. The rest of the water will need to be treated with a chlorine remover. The easiest way to move your fish is to drain the pond (removing the pond equipment) until there's just enough room for your fish to swim around. The fish will be easier to find and scoop out using a skimmer fish net. As you scoop out your fish, place them in the container (make sure they can't jump out).
Acclimation
It's essential to get your fish used to their new environment slowly. If you take your fish straight from the pond into their new indoor tank, they can become shocked and stressed (unless you only used pond water and just set up the indoor tank). Check the water temperature of the temporary container and then the indoor pond to see the difference. If there's only a 15°F difference, you can do the transition in an hour, but if there's a big difference, you need to take it even slower over a couple of hours.
You can start acclimating your fish by slowly adding water from their new tank until the water temperature is the same. Besides the temperature, this will help the fish get used to the other conditions of the new water. Add 10 percent water to the temporary container from the indoor overwintering container every 15 minutes (wait 30 minutes or more if there is a dramatic temperature difference). Once the water temperatures are the same, you can add the fish to their new indoor tank. If using the bag method, sit the bags on top of the new indoor tank's water until the temperatures are the same.
Water Temperature
Fish are cold-blooded, so they require heat from their environment. The colder the water, the less active they are. Their digestive systems and immune systems slow down during the winter if they are outside. If your indoor tank is too warm your fish will be active, needing more food and excreting more waste. Water quality can become an issue quickly and require frequent water changes (around 20 percent every one to two weeks). Even if the water is kept below 50°F, you may need to perform water changes.
Hardy fish like koi and goldfish can handle colder temperatures. If they were to overwinter outdoors in a large pond, they would swim to the bottom, where the water is the coolest. Once the water temperatures drop below 50°F, the fish do not need any food and are not very active.
Many pond owners prefer to overwinter their pond fish in indoor tanks that are kept below 50°F but above freezing. The water should not start to freeze on the surface. You can put your tank in a protected area that will not get too cold and will not keep the water too warm, like the basement or garage. It's essential to pay attention to the water temperature, as it is often different from the outdoor temperature. Because these tanks have less water than your pond, they can quickly drop in temperature and start to freeze.
Maintenance
Once you've successfully moved your fish to their indoor tank, monitoring for any signs of changes is important. Be on the lookout for any sick fish so you can quickly isolate them from the tank or treat all of your fish. Regularly perform water checks once a week or at least bi-weekly to see if the water is balanced. Ammonia and other contaminants can quickly spike and harm your fish in the smaller environment. Perform water changes as needed, and be sure to use a chlorine remover. If the water temperature is below 50°F, your fish will not need any food. Learn more about feeding your fish in colder temperatures.
Overwintering fish indoors is a must if your pond is too shallow or small and will freeze all the way through. Hardy fish like koi and goldfish can overwinter in an outdoor pond if it's deep enough that there will be unfrozen water at the bottom of the pond and a section of the ice is kept open to let the water oxygenate. Fish can be moved indoors into a holding tank, aquarium, or another fish-safe container and kept in the garage or basement for the duration of winter. Ideally, the water is kept cold but not freezing (around 45°F), so the fish are less active and do not need food. Once your fish are set up indoors, the goal is to make sure there are no changes and the fish are kept healthy until spring, when you can reopen your pond.
