Temperature Control....gotta loose some heat

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Heat from the lights is going to heat the surrounding air, but not the water so much directly. Blowing fans at the lights will distribute this heat into the airspace surrounding the tank, but I don't think it will be too effective at actually cooling the tank. Eventually, if all the air surrounding the tank/sump gets beyond the tank temperature, it will start to heat the tank, though.

...unless you've got a really closed-in light canopy that traps a bunch of hot air directly above the tank, then this effect could be greater.

I think the bigger effect is the energy of the light hitting the rocks heats up the rocks which then heats the water. If fans on the water surface won't do it you are going to be really pressed to avoid some sort of water chilling.