I fasted mine for 5 weeks, during which 59F was the lowest pond temp. Most days the pond was in the low 60sF. Algae on the pond walls has never been grazed any shorter.

I cannot say there was much weight loss, but some slimming occurred. After about 10 days the fish stopped coming up to me for food. They learned there would be none. They were gradually fed increasing amounts over the past two weeks, primarily Hikari sinking wheatgerm. They are now on full rations, with some Hikari staple floating food beginning to be introduced.
BTW, the cold front that came through early this week brought the lowest pond temperature of the winter. Down to 56F on the morning when air temp was 35F. I left the Bakki uncovered to see what would happen. There was only a 4F 12-hour drop. This morning at 7am, the pond was 64F.