[cbkhoon]

since u have lighting over the tank, the temperature inside the tank will be slightly 0.5-1C higher than outside. You may try to mount the fan on the surface of the tank, sucking hot air out, cold air will go into the tank automatically, hence improving ventilation. At the same time, you may need to check the humidity inside the tank with a humidity meter, take note at the reading during the fan is on and off.

You may also blow air into the tank by reversing the fan, and again. check the humidity meter to see which type of installation is best for u.

just my 2 cents