I'm actually facing the same problem. Do you have a basement or underground room where you can move the tanks? At the very least, avoid placing them in a room on the top floor of the house. I live on the top floor in my apartment and it gets brutal in the summertime.

Air conditioning is the next step. Try to arrange the tank and the AC close to each another. Fans are a good idea anyway. A small 2.5 inch computer fan attached to the terrarium will generate air flow necessary for tree frogs (and many plant species) at any temperature. Do mist often using room temperature water. Cold or icy water will shock not only the frogs but also the plants in the tank.

What you can do is place chilled bottled water, ice cubes or cold pack on top of the vent, and blowing a fan across it into the terrarium. You can also cover it with something made of insulating material like a plastic bucket. Lastly, you can place a small bowl/dish of chilled water (50-60 degs F) inside the terrarium, changing it every hour as needed.