Well, I have one of my setups where the fan (120v/ac- computer tower fan) blows across the 250w/mh bulb and there is very little temp spike in the 50g tank it rests upon (about 1-2 degree temp change from when light is on/off). In another set up, I have a small (7") house fan the pulls air up and out of a canopy that houses 2-250w/mh lights, which hangs over a 110g tall tank. There is more of a temp rise in this tank though, 2-3 degrees, as the tank is closed and there is little to no evaporation of water, the 50 gal top is halfway open, thus more cooling/evaporation. I prefer to blow across the bulb and water surface, rather than pull air out of the canopy, but this is just how I feel, no scientific evidence other than temp ranges for ea. tank... Hope this helps...Erick....