If you have money to burn, you can always buy an aquarium chiller... the cheapest would cost you ~ $300.
I couldn't afford to buy a Chiller.... so, I bought an Aquatek Aquarium Cooling Fan from
Aqua eWorld on eBay. This fan has been a BLESSING to my tank during the hot summer months!! Even though i have a canope, the air flow from the fan drives out the hot air accumulated inside. The little clips on the fan snaps onto the aquarium wall very easily.
It dropped the temperature of my tank in hours and saved me $300!!! (and save energy cost too!)
you can give it a try. Good Luck!