What is the ideal temp for goldfish?

I was wondering what the ideal temp is for goldfish, and how low can the temp get without harming them?
My mom has a pond (not sure how many gallons, but it's in their flower bed, dug into the ground....just got finished this summer) I'd say it's about 2 feet deep (just a guess)
Is it ok for her to leave her fish out over the winter, and live under the ice...or would she be better off to bring them in? She'd like to bring them in but we were just curious.
I also let my mom take my 3 goldfish from the fair as I read on here they don't do well in small aquariums. I agree, they seem much happier there. It seems they taunt my brothers cat, he'd love to get ahold of one, but the way it's set up he'd fall right in if he even tried, so he just peers over the edge. Thanks for any replies
-Mandi

 

I dont think they would survive a wisconsin winter. our temps here dont get below 30 and our fish do okay in our pond. they do have pond heaters but again it get so cold there I dont know if that would work either

 

I'm not 100% sure about Wisconsin. The temperatures in Missouri generally get down in the 20's (sometimes lower) and people have told me that they winter goldfish in 2' deep ponds just fine (I made mine 4' deep to be on the safe side). Wisconsin gets a whole lot colder, though. Probably your best bet would be to look around for a pond store in your area and ask them.

-Chelle

 

More than likely a 2 foot deep garden pond is going to freeze solid in a Wisconsin winter. It will get below freezing and stay there for a good long time in. You can anticipate temperatures below zero many nights, and not a whole lot warmer during the day.

If you want your fish to survive, it would be best to bring them inside for the coldest months.

 

how deep do you think a pond would have to be in canada in order to keep them out during the winter
it gets about -30celcius during the winter and doesnt ussually get up above -10celecius all winter