Take a clean cloth with a little white vinegar on it and rub the calcium deposit with the vinegar making sure that none goes in the water. The calcium will dissolve and you can then polish the glass with dry kitchen paper towel.
Please be carefull to do this only on the outside of the tank as the vinegar will affect the PH of the water if it gets into it
I own a pet store and we clean the outside of the display tanks this way..