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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, so I have also some baby gold fish that I'm trying to give away.

I live in northern Italy where temperatures in winter often go down to -10 °C, water in a container above ground usually freezes for about 10cm from the surface.

I have means to not let the water freeze too much, in a way to always have some liquid water exposed to the air.

My question is: Do low temperatures affect the adult fish? What about the babys?

Fish health is my priority so if I need to get something special to ensure that my fishes are healty, I will have no problem getting it.

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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:

  1. Be sure not to feed your fish if the temperature is below 10 °C

  2. Stop the filtration/pump when the air temperature is below about 6 °C to limit the cooling

  3. 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.

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  • Aeration is another way to keep some opening in the ice for O and CO2 exchange. Gradual temperature change is important and is normal in a pond. Giant and zebra danios ( tropical fish) survive in my pond to water temperatures below 52 F about (11 C) , I think the gradual change is important.
    – blacksmith37
    Commented Nov 6, 2019 at 16:02
  • @blacksmith37 i do not mention aeration in my answer because the moving water might lead to the temparature going down more than one might want,cold water will hold a large amount of dissolved oxygen so there is no need to add more to a pond,as long as the CO2 can escape the fish will be fine.and gradual temparature change is important,one can put styrofoam on top of the water to insulate the pond so the temparature will drop slower during the winter months.
    – trond hansen
    Commented Nov 6, 2019 at 18:30
  • I have suffocated large koi covering 90 % of the pond with Styrofoam ( it did not bother the danios) so I l now use the aeration . But ,I agree stirring the water probably cools it more.
    – blacksmith37
    Commented Nov 6, 2019 at 22:53

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