Your question depends on many things. Here are a few right off the top of my head and I'll address my thoughts and give ideas.
1. How high are the lights above the surface of the water?
2. Is the canopy well ventilated?
3. What type of lighting is being used?
I've found that the closer the light is to the water, the more problems you'll always have with heat when the temps increase. This is especially true with power compacts and even worse metal halides.
I'd recommend raising the lights, providing space around the "reflector" for ventilation and if possible placing a small sheet of glass directly underneath the light covering approx. half the surface area of the top of the tank.
Then if all of those things are being used and still not keeping the temps down, try placing a small computer fan pointed directly at the bulbs, not the water. Then if that doesn't work, or it makes too much noise, or it evaps too much water indirectly--because you pointed the fan at the bulbs right?--then try for a chiller. They're mad expensive.
Another thing to consider would be to run your lights on a reverse lighting schedule ie, when it's night your lights are on and during the day when the room temp is the highest, keep the lights off. This is an excellent way to maintain good tank temps.