anything between 12°C - 23°C. with something in the middle probably being the best.
I have heard I think that 18°C is a good healthy medium.
as for fahrenheit I have no idea sorry.
They'll tolerate high 20 C range provided the water quality and aeration is good. Lots of quality food availability, along with warmer water makes them grow fast if that's what you are after. Of course in a tank setting, this means more frequent water changes, etc.