I do not think that anyone knows for sure.
I have had temporary issues where the water got colder than that but, never for a period longer than 24 hours.
I think that the rays will be fine. Their growth and metabolism will slow down but, they will be fine. I am sure that there are systems in the amazon that get much cooler at certain times of the year than the 80+ degrees we keep them at regularly.
As a matter of fact, I will be shocked if they do not do well for you.
Colin