Average temperature in Tel Aviv right now is 75 f, which would explain why they can’t take the cold.
I know folk here who slowly acclimatise Koi who are used to high temperatures by putting in a heated tank and dropping the temperature very slowly. Say by 1f every couple of days and stopping or raising slightly, if the fish show signs of distress until they stabilise, then starting again.
This seems to work 90% of the time as it’s a much reduced shock to their system.
Main pond 4000 US Gallon, 22 Koi. Oase Proficlear Premium + Bio Module, Bitron 120 w UVC, Bakki Shower, Dura 7+ ashp. Grow on tank 600 Gallons with Eazypod Automatic and 70 litre K1 biochamber.