Hi Lucy. Here's a link to some chillers, but they are rather expensive so I don't know if it's an option for you. Perhaps they have cheaper ones at your lfs.
Otherwise, I would say AC, but since you don't have one - a fan would probably help cool it off a little.
Other than that, ice cubes are really the only other thing, but I wouldn't recommend this unless it's completely necessary because then there will be icy cold water portions of the tank. The fish won't like that.
Chillers

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Phishie

"An ounce of prevention is worth a pound of cure."