In order for the fan to cool the tank it must blow across the surface of the water -you cannot have a cover on the tank, which would block evaporation. When water evaporates it uses up 596 calories of heat energy, lowering the temperature as a consequence. If the tank is covered so that airflow is impaired evaporation will be reduced, so cooling will be minimized. I use plastic egg crate on my reef at work to allow evaporation, and it never gets above 79 degrees, even with 3 submerged pumps and two powerheads (and the room temperature around 75-77).