I run a couple of 400w(and a couple of VHO antics) with a couple of hood fans and my canopy is fairly open (no glass on top of tank) and I still struggled with temp increase, 2- 3 deg by end of day, so ended up putting a small house fan over my sump - all set up with timers. Seems to be doing the job so far.

My tank temp was going up by 4 degrees during the day. I put a small computer fan (3 watt) blowing between the light and the water and don't see any increase at all now. I don't have a glass top on my tank, so I see a fair amount of evaporation.

Evaporation = free cooling! :)

I'll second that!

How did you power the computer fan?

I used a couple of small 12vdc power supplies i picked up from 'Princess Auto'.
Works like a champ.

I actualy had one kicking around that's 120V. I put a reostat on it to be able to change the speed. Right now it's set very low, but if needed I can speed it up to get more air moving across the surface of the water. :)