: Gold fish survivability in winter I have a small pond, about a meter deep with some regular gold fishes in it, nothing fancy. The fishes look healthy, they have a really good color, they are active and they recently reproduced,

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Goldfish do not have any problems surviving the winter as long as the water at the bottom of your pond stays above 2 °C.

As far as I have seen, young fish are not more sensitive to the cold than the adult fish are (though they might lose a little bit more body mass during the winter than the adult fish do, but this is not a problem).

There are several things you can do to be sure your fish survive the winter:

Be sure not to feed your fish if the temperature is below 10 °C
Stop the filtration/pump when the air temperature is below about 6 °C to limit the cooling
Keep an open hole in the ice (you can use a piece of Styrofoam with an aquarium heater in the middle to keep a small area open); this is to let the CO2 out of the water and to let some oxygen into the water.

You need to know the spring will be the hardest time for your fish; this is because the immune system of your fish will be less active until your water temperature gets above 10-12 °C (so you might experience the loss of a few fish when the spring comes).

I live close to Oslo, Norway and have had my pond for close to 30 years. It is impressive to see the fish in the spring and that they have not lost weight during the winter even if they have not eaten in several months.