Many folks will use a clip on fan - should be able to get one at any drug chain (CVS, Duane Reade etc...). These will often lower the temp by a couple of degrees, but they will also increase your evaporation rate.
Depending on the type of lighting you have and where your tank is located in your home (basement tanks run cooler), you may still either need a chiller or a whole room A/C to keep the tank cool enough during the summer months.
Some folks who have a controller on their tanks set them so the lighting shuts off when the water reaches a certain temperature.