At 13x10x only 1.5' your looking at about 1400+ gals. Much more than the 800 you figured. Go at leas 2' deep. At that depth, even in USDA Zone 4, which in Minnisota, all fish willl survice the winter provided a hole in the ice is kept open. Don't worry about any low temps. for natives, they are in the frozen lakes around you now aren't they?