When it gets up to 100 degrees, I run the air conditioner to keep the house cool. It then in turn keeps the aquarium cool. If the air around your tank is at a constant lower temperature, the tank will stay somewhat close to it unless there is a large heat source near it...which you can see from people who run MH
lamps over their tanks. Those are the people who normally require chillers. If running the AC
isn't a viable option, then a chiler might be a viable option for your system. I personally try to avoid dropping so much money on a piece of equipment that I already have going to make sure my dog doesn't get heat stroke while I'm at work...chillers are expensive!
If you run the AC
and your tank is still warmer than you want it, try a small desk fan clipped to the side of the tank pointed at the surface of the water. This will bring the temp down some...as long as the air around the tank is cooler than the tank itself of course!