I started my whole fishkeeping hobby with pond goldfish in a 10 gallon tank for the winter. The first season I lost all 5. Goldfish are sooooo dirty and keeping them in a 10 is just too much work. I have trouble with them in the 30 over the winter...keeping the tank to where it should be. Water changes just need to be done so often...they're worse than Oscar! LOL!
If it's a good size pond...deep enough...you should be able to leave them out for the winter. They slow down when the water gets cold and almost go into a 'hibernation mode'...they can live under the ice...just as long as the pond water won't freeze solid and make little fish-cicles...
