should be considered in two places
1) for your canopy to dissipate the immediate heated air around the halide bulbs and removal of the heated air out of your canopy away from your water
2) fan aimed at the water because the process of evaporation uses up a lot of energy (consumption of energy required to bring a liquid to a higher state [in this case vapor] is significantly higher). as more water is evaporated, it consumes more heat (energy) which brings the water temp down. I have a small 4" fan that maintained my 250 gallon system at 81 F over the summer. this was before I installed the MHs but nevertheless, it was at least 88-90F for many days in the summer and my tank kept its cool around 81.
hth
cwlee