Fish can be moved indoors into a holding tank, aquarium, or another fish-safe container. Even a small swimming pool in a garage can sometimes work. Ideally, the water is kept cold but not freezing (around 45°F). This keeps your fish in a state of hibernation, so that you don't need to feed them.
Make sure that you're providing enough room and the right water conditions for your fish to thrive. Since your fish won't be eating, they won't be producing waste, so you won't need to change the water as long as everything is functioning as it should. Invest in a water testing kit so that you can keep a close eye on the condition of the water your fish are living in.
