When I picked them up at Mystic Koi the water temp at the facility was sitting at 50 degrees because most all of the ponds are under heavy shade cloth. I doubt the fish were being fed at that temperature but would need to ask to crew at Mystic to validate that statement. At 9pm I will speculate.
Tosai will live at 40f but if you want them to amount to anything or even live they should be kept at a minimum tem of 68f the first winter . If you really want to grow them then 75f would be better . Growth is largely determined by genetics but health wise I would not go below 68f
new tosai should be kept at 68f for at least the first 6 weeks in your care or you could be asking for problems . Wish it was otherwise as it is costing me and main reason why I only get tosai every few years .
Regards
Eugene