Heat is not as much of an issue as people make it out to be. I had 6 watts of gallon VHO
lighting at one point lying on top of the glass that covered my aquarium (I did not have a canopy and got retrofits for free). Even with this gross transgression I did not overheat, because I kept my evaporation high. My suggestion is to try it, and monitor the temp (mine rose to 76 from 72). If the temp stays below 79 (as mine is now with metal halides), all you have to do is set the heater to the max so you do not get fluctuations (its not so much the high heat that kills corals but the day/night fluctuation that does, as the ocean only fluctuates +/- 5 degrees between winter/summer.) If it does go higher, you still have PLENTY of things that you can do before resorting to a chiller. First, place a fan in your stand. Yes thats right the stand. It is MUCH better to put it there than in the canopy. If you put a fan in your stand and leave the top of your sump as uncovered as posible, what you will end up with is a lot more evaporation. and evaporation is the best cooling mechanism for ANYTHING. Most mammals do it, and we are even using evaporation to put out fires, and cool down nuclear power plants. If its good enough for a nuclear power plant, its good enough for our reefs. Finally, if that is not sufficient when (and I do mean when, not if, because everybody who is serious about reefs eventually does) get metal halides, if that does not work, try put one or two into the light. With that combination you will doubtfully ever overheat.