I had the same concern Lori. I fought with it while the rock was curing. The end solution wasn't as hard as it seemed. I wanted to see how warm the tank could get with the MH's on. I ran the tank without lights with my heater set to 75, the temp read 75.5-76 for three days with no light. I then turned on the MH's and let them run for 10 hours (which is how long I would keep them on for). The temp rose to 81-82. I set my heater to 79 so the temp read 79.5-80 with the lights off for the next two days. I then turned on the lights temps rose to 81-82.
I ran the tank like that with a 2 degree shift in temp from sun up to sun down. Not bad at all. I wanted more consistency so I started aiming my fan at the tank which has a built in thermostat. When the ambient room temp rises over 73 the fan turns on, when it reaches 71 it turns off. Since doing that my temps dont move from 79.5-.7. I have seen more evaporation in the tank since the fan but nothing major.
All in all its not that big a deal. You can get clip on fans from walmart or there are lots of stores online which offer great clip on fans which clip on the back of the tank and help expel the heat. Cheap alternative to my fan which cost me around 80 bucks. But that was when I needed a fan for me not for the tank. I just used it cause I have it sitting there anyway.