I moved my other Zebras back into the outdoor pond a few days back. They are doing well. I think their limit is around 50 degrees. Any lower and they struggle.
6 weeks later, May 6 and the Zebras, Least Killifish, Flag Fish, even a couple of Rainbow Cichlids, all doing great. And the Water Hyacinth that was dormant over the winter is back growing like crazy.
The tennessee Aquarium had least killies in some of their reptile display water areas and according to one of the staff I asked they do not have as large a population during the winter as they do in the summer even in that space. So the winter time decrease seems to be normal in a unheated environment.