Your photos give me goose bumps.
The lowest water temp I measured is 54F, but I believe my 1000 g pond got to about 50F. I float house sheathing foam panels on it during cold spells ( air got to +18F for a short time). I have about a 70 watt pump and I occasionally used a 150watt heater in addition.
It is warm now (62 water) and I have been trying to get the last of the swordtails out ; got about 20 in the last couple days. There are still about 50 in the pond. BUT : with the swords ,I netted 5 more zebra danios; tough little fish. They bred in my pond system last summer.