Interestingly, you can actually allow most of the pond's surface to freeze over and still keep your fish perfectly safe. The hole in the ice doesn't need to be too big in order to allow the oxygen levels to stay high and give the fish the gas exchange they need to breathe comfortably. In general, most de-icers will make holes of about 10 inches in diameter, and that should be just right in most cases. It all depends on the conditions, however. If you live in a really frosty area, you'll need to buy a more powerful pond heater to make a big enough hole, while those with milder winters won't need to worry too much about hole size as the pond shouldn't freeze too easily.