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You can draw any curtains/drapes in the room during the day to keep heat down.
You can run a fan across the top of the tank to draw the coolness into the water column.
Please do not add ice cubes. This can not only change the temperature too quickly, it can also mess with your water parameters as this is now chlorinated water.
Ive used ice cubes in the past. But keep them in a zip lock bag. That way as the ice melts, the water does not get into the tank. Unless you make your ice cubes with ro/di water, than it doesn't matter