The pond is deep enough for goldfish, but something usually eats them during the summer... I can put a ball or a heater in to keep a spot thawed for oxygen if necessary. It is 18 inches deep and about 175 gallons, below ground in climate zone 6b. Also it is nearly choking with winter hardy oxygenating plants, like elodea and coontail. As long as they are supposed to overwinter underwater, as your fabulously succinct reply states, I suspect he will be fine. Thanks guys!