78*F water temperature will feel cool to us as it is 20*F cooler than our bodies.
Larger volumes of water hold temperature better than smaller volumes.
Keeping the lights off will lower temperature. Keep the tank in a spot that is out of the sunlight and where it will get air currents. (towards a center spot on the wall or room, rather than tucked into a far corner.
Adding an airstone can also help lower temperature as it encourages evaporation. Surface movement in general will help with this.
That all being said, I do not feel that your ADF will be endangered long term if you have slightly warmer than average temperatures.