Warm air can hold more moisture than cold air. When the warm, moist air comes into contact with cool air or a cool surface, the warm moist air cools and loses some capacity to hold as much water and as a result there is condensation.
If you want to reduce condensation you have three main options:
1) reduce the amount of moist air in the tank that leaves (in short, increase the area blocking the exit by adding a top or by making the top better sealed.)
2) reduce the temperature of the water in the tank, thus reducing the capacity of the air in the tank to hold water and thus reducing the amount of water transported out of the tank.
3) increase the temperature and humidity of the air around the tank (i.e., the room) which will reduce the rate at which warm moist air in the tank chills and causes condensation, especially near the tank.
Of course any one of these, or a combination of these could help, so one can pick and choose.