I agree with the above it depends what is causing the heat in the first place...
If ambient room temp is high, then a fan isn't really going to do too much ime... you might get lucky and have it drop a degree or two, but not too much below that... not to mention, you can't control how much it will cool down the tank...
Chillers, especially for larger tanks, is pretty much one of the only ways i know of to keep a tanks temp down and stable... other methods such as keeping ice-packs in the tanks etc do work, but only for a short duration and naturally causes a lot of temp fluctuations...