Ideas for keeping tank cool

Minimize the light period, have fans available. Monitor, make sure temps aren't spiking too high--do more frequent water changes. Dropping the water level a bit will increase evaporation (and cooling), as well as increasing gas exchange (warmer water holds less oxygen). Using cooler, more efficient lights (self ballasted CP are great) instead of hot incandescents, as well, though might not apply to your setup.