you need a controller for the fan or just put it on a timer to coincide with the hottest part of the day/halide. You might be able to get a cheap used ranco controller if you post a WTB.
78.7 is not that high at all, unless you have cold water stuff. In the summer, my tank gets to right at 80 and I don't worry about it at all.
I second a controller to turn those fans on and off. It is the only real security, or hook them up to the same timer/switch as your lights. Easy way to turn them on and off.