I run the same setup on a 24 gallon nano cube. I found that the temperature would swing by 8 degrees and worst on really hot days. Now, I no longer have to worry about it. I would highly recommend that you purchase a controller such as the Digital Aquatics Reef Keeper lite. Have it hooked up to your HQI, heater and tank mounted fan.

I would also suggest an Auto Top Off as the fans will cause a lot of evaporation.