hi have you got a heater this should be keeping the temp stable fish shouldnt have more then 1'c temp changes it stress's them i would suggest getting a heater or a new one (if u have a heater it sounds faulty) but yeh it would be safer getting a new heater
for your second question bettas thrive on heat mine are at 83'f he is happy healthy and more active then having the temp at 79'f but it should be fine mate any other questions?
(but to answer your first question yeh a little to stressful