I am only a novice fry hatcher, but I decided to throw in my two cents ;-)
The room that I use to hatch my rainbow fry for some reason is the warmest room
in the summer and the coolest room in the winter. We are currently having
extreme weather here in NC. For the last three weeks highs have varied from 85
to 45 F and lows have varies from 50 to 20. This has caused my fry tank to
vary in temperature because the cheap heater I use can't seem to keep up with
the crazy weather. The temp in my fry tank has varied from 76 to 84 and the
fry seem to be fine.
John Dowden