I have had big outdoor ponds in the past, but now all I have is an 800 gallon holding tank in my greenhouse. It keeps the temps in there more stable and helps keep the humidity higher. It has a solar aerator which only runs when it's sunny. It doesn't have any other filtration or heating. Right now the water is around 80 degrees, but it goes down to about 55 degrees in winter. No ice, due to a small heater in the greenhouse during the winter.
That's great that your fish all survived the winter. I'd love to have nicer fish, but would need to add more aeration, filtration and heating. Here are some pics from last year:
