Goldfish are cold-water fish that are accustomed to temperatures between 60 and 70 degrees Fahrenheit. When water drops substantially below this level, they stop eating and go into a state of dormancy, so providing proper care during cold weather is vital for keeping your fish alive.
If your goldfish live indoors in an aquarium, they are unlikely to need special care during the winter. However, if you keep your house very warm, check the water temperature and consider moving your fish to a basement or other cooler area of the house. Goldfish can become lethargic or ill when water temperatures rise above 75 degrees Fahrenheit. Conversely, if you keep your house very cold -- below 60 degrees -- invest in an aquarium heater to keep water temperatures stable.
When water temperatures remain below 60 degrees for more than a few days, goldfish enter into a period of dormancy. They slow down, stop eating and hover at the bottom of the pond. Goldfish can survive several months of hibernation, but you should feed them slightly more than usual in the months leading up to the winter. When the temperature cools, begin feeding less. The temperature must drop gradually for fish to adapt. Never move indoor goldfish outside during the winter or place goldfish outside after you've moved to a cooler climate.
In colder temperatures, particularly if there is ice in the water, water oxygen levels drop. Goldfish need adequate oxygen to remain healthy, and if oxygen levels are too low, they may suffocate. Provide your goldfish with oxygen by adding an air stone designed for your particular pond size. For example, don't use an air stone intended for a 25-gallon tank in a 200-gallon pond.
Goldfish can survive even if the top level of the pond ices over. However, it's helpful to leave a small hole in the ice to maintain oxygen levels. If the pond ices completely, your goldfish could freeze. If you live in a very cold climate or your pond begins to thoroughly freeze, install a pond de-icer.
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- University of Illinois Extension: Koi and Goldfish Pond Winter Care
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