It means the glass is cooler than the water, so the evaporation from the tank is condensing on the glass. Unless you want to turn off your air-conditioning, I don't see what you can do. Most of the glass defogging products (live for eye-glasses and windshields) aren't safe for the inside of a fish tank. Do you have a specific reason to keep the water level low? Like live plants sticking out, or to lower the temperature? If you don't, just fill the tank to the bottom of the top trim and you won't see the fog. Change water once a week and top off the tank in between.
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