Ohio...A 4 degree swing isn't terrible but if it increases you could be stressing the corals and/or fish. A second fan in the canopy might help, it sounds like the existing fan exhausts, adding a fan that blows in from the other side could help or fans on top to exhaust heat upwards. Its hard to say without seeing it or knowing more. Fans blowing over the sump or the tanks surface would facilitate evaporative cooling but as you've said that would increase your rate of evaporation. What do you think is causing the rise in temperature, the lights or the room temp. A simple solution could be to AC
Fishman...a 2 degree temp swing IMO
isn't bad at all, actually compared to what I've read from others its actually pretty good Especially in Texas.