I second BJ's idea.. with 900w MH over our tank remp were getting unreasonably high, 84+, the addidtion of a 2 4" fans blowing under the 250w bulbs onto the water and a 9" desk fan under the 400w bulb holds the temp dead on at 78 with just the actinics and 80 max with the halides running all day. Keep in mind evaporative cooling while being a cheap way to keep temps in ckeck will increase the tank's daily evaporation so you'll need to mind your water levels a bit more. But $5-10 for the fan(s) sure beats $600 for a chiller, and costs a lot less to run (I just have mine on the timer circuit with the lights.

You said your canopy is vented, can you elaborate a bit, fans in the sides or top? Is the back sealed or open?