Dragon,
We don't get quite as cold here in the mountains of WV, but our ground still freezes down at least a foot or two every winter. We typically get as much snow, and sometimes more, as Wisconsin's Lake Superior south shore.
We had a very similar pond to yours for a number of years at our previous home. One springtime I put some small ten-cent goldfish in the pond. By October they were about 3-4 inches long. I knew they wouldn't survive the winter in the small pond. We found what I believe was a 50 gallon aquarium at a yard sale. They spent two winters enjoying that aquarium.
I learned a lot from that little pond. I learned that frogs have a natural anti-freeze in their blood. They just go dormant in exceedingly cold temperatures. We tried to aerate. Instead, the little pond just froze to the bottom. We never had a shortage of frogs once the water was mostly ice free.
My son has a 300-400 gallon aquaponics tank where he raises tomatoes, cucumbers, and salad fixings from very early spring to about Christmas. He's got gold fish in there that are now three winters old and are about 10" long, along with some younger ones that are 4"-5" long and some from this season. The setup is built from a 4' x 4' x 4' Universal Bulk Container. He put 2" blue foam insulation around the sides two years ago and added a floating cattle tank water heater from Tractor Supply. I don't remember which heater he purchased, but it is similar to the ones found on this Amazon page:
Cattle/Equine Water Tank Deicer Heaters He leaves the pump/siphon run year around. He covers it in the winter with black plastic over garden hoops. Gold fish from the last two seasons survived our winters. Our temperatures typically get down to between -10 and 10 degrees F in the winter.
I have a low hoop house hotgarden conduit bender like this
Low Hoop House Conduit Bender You could also make a low hoop house out of inexpensive PVC pipe covered with inexpensive painters' plastic sheeting during the worst of the winter.
Good luck.
