They need oxygen in the water--hence the advice for aeration and pumps.
Our pond is 20' x 80' (approx) and spring fed. It is 9 feet deep and you can see all the way to the bottom. We don't feed our fish and our pond freezes over pretty solid--we live in a valley so it gets -20 to -30 here in the winter but the spring keeps fresh water coming in all winter. We only lose one or two to natural causes every winter (herons, kingfishers, osprey, eagles and cats claim some in warmer months)
Some friends of ours also have a pond but theirs does not have a spring. At the end of a long winter, they find quite a few dead fish in theirs. They poke holes in the ice in early and late winter and it seems to help. Like everyone else in N. MI, they have ice fishing equipment and use borers for that.