That's a very nice setup! I would love to have something like that myself, but here the problem is not winter. In the summer it can reach 118 or even higher.
Congratz on a job well done! you actually did something that I was thinking of doing for a long time lol. burrying a pond half way into the ground and using concrete all around. Do you have a lid for it during the winter? That was my idea for outdoor heating of a pond in the cold winters of montana. I have found with a tight lid (vented for transfer of gasses) that it does not take very much to keep a pond very warm outside in the winter. The ground makes a great secondary insulator against the concrete. please keep us posted on this amazing pond.