The real question is what do you want to cool?
If you want to keep the water cool, you should have a fan pointed at the water. This eliminates the layer of vapor on top of the water. If this is gone, the water will keep evaporating and therefore cooling the water.
If you want to cool the lamps, then aim the fan there.
I think a fan sucking air out will not do much good at cooling the tank. Like electricity, air will follow the path of least resistance and most likely leave the vapor layer on the water.
Your best bet would be one of each. A fan aimed at the water and one on the opposite side to help exhaust the excess vapor and eliminate swirling.
Jim