Hmmm... it's a small tank, so the ice cube method might not work so well. What if you put the tank in a cooler place in the house? Like the bathroom or some other cool room? What about pointing the fan directly on the tank?
I know when my 10 gets too hot I float a bag full of ice cubes in the water - but because your tank is so small, I'd be afraid that the ice would make it too cold too fast.