i only use heaters during the winter time, my tanks just ride the temperature and my fish dont seem to mind at all, today my 6 foot tank is at 20 degrees C, thats 68F, my fish dont mind at all because they are used to it.
i am in the uk, its freezing right now. i use heaters in my 3 and 2 foot tanks, they are kept at 26 degrees celsius. i dont use a heater in my baby tank, its only 5 gallons, and the light keeps it at a steady 25 degrees
I only heat my growout tank to 78-79F. All my other tanks are heated by the house temp and lights to around 76-77F. My fish stay healthy and very active.