Moving koi or goldfish indoors for winter is an option, not a requirement for every pond. A properly prepared outdoor pond may support fish through cold weather, while an indoor holding system introduces its own water-quality and handling risks. Decide based on your fish, local climate, outdoor pond, and the indoor system you can actually maintain.
When staying outdoors may be reasonable
Look at the pond's depth and design for your climate, whether it could freeze through, and how it maintains circulation and gas exchange. Remove excess debris before winter and follow the pond equipment's cold-weather instructions. Do not assume a single depth or de-icer arrangement suits every location. For the wider shutdown decision, see Tips for Winterizing Your Pond.
What an indoor setup must provide
An indoor tank or holding pond needs enough usable space for the fish, suitable filtration and aeration, reliable power, and water that can be monitored and maintained. Plan for where the holding system goes, how you will contain splashes, and how you will respond if a pump or filter stops. Indoor water does not become safe merely because it is warmer or easier to see.
Consider the move itself
Netting and transport can stress or injure fish, particularly if they are handled abruptly or moved between very different water conditions. Plan the transfer with the receiving system ready first, follow species-appropriate acclimation guidance, and seek experienced fish-care advice if the fish are unwell. See How Do I Transport, Net, and Move Pond Fish Safely? for handling considerations.
Monitor through winter
Whether fish stay outside or move indoors, watch their behavior and water conditions and maintain the system as required. Feeding depends on water temperature and the fish's condition, not simply the month or whether the pond is indoors. Winter Fish Care covers seasonal feeding. If you cannot provide an appropriate indoor system, moving fish may create more problems than it solves; ask a qualified pond or fish-health professional to help assess the options for your specific pond.