As long as there is some room beneath the ice, goldfish are able to survive a frozen pond as long as they're properly prepared and the pond is winterized. Goldfish, like other kinds of fish, survive the winter by entering a period of dormancy where their metabolism significantly decreases and they don't require as much food as usual. During this time, they expend less energy, and rely on their fat stores for necessary calories.
You'll want to begin removing debris, sludge, and muck from your pond in late fall. This will reduce the amount of decaying organic matter present once ice begins to form. Decomposing organisms and the gases they produce can be very harmful if trapped under a layer of ice. You can also prevent this by keeping part of your pond from freezing over completely. Not only does this allow gases to escape, but it allows oxygen to access and permeate through the water.
