Yeah, I kinda thought that heat might build up in such a small setup.
The problem is that you have MH lights giving off 70w of heat, which is both radiant and convective. You need to move a massive amount of air to cut back on the convective heat (which also increases evaporation), but they don't do anything for the radiant heat cooking the water.
Water has a tremendous heat capacity:
Q= mc(T2-T1)
Q = heat energy
m= mass of water (kg)
c = specific heat of water at 25d C
T2 = Ambient temp (C)
T1 = Water temp (C)
...so it holds heat very well. Getting rid of that heat requires just as much energy plus some (no system is 100% efficient).
Fans don't actually COOL anything. They just move air. The cooling effect is caused by evaporation of moisture off your skin from the constant flow of air (evaporative cooling). Or in your case the displacement of hot canopy air with cooler room air. Eventually the ambient room temp will absorb enough heat to equal that of the canopy and then get warmer. (Though this may never happen if you run air conditioning, cause the a/c is removing that heat).
Also with Newtons law of exponential cooling, the rate of heat change is exponentially higher the farther the temp differential. This is why a cup of coffee goes from being extremely hot for a few minutes to staying warm for half an hour. As the temps get closer and closer together the rate of heat transfer slows down.
So a difference of only a couple of degrees between your desired tank temp and the elevated temp will take a long time to even out to room temp. Combine this with the fact that you're also constantly pumping heat from the MH's into the system and it simply won't happen.
Sorry to say but the only way I can see for you to reduce the temp is to either reduce the amount of heat generated (switching to PC, T5's, or T8's) or getting a small chiller ($$$$). A temporary fix would be to flip one fan so it sucks air into the canopy, and the other so that it blows away. This will create a flow of air in to absorb heat and then quickly pump it out. The more directed it is the better. But it might only give you a degree at best.
You could make
make your own chiller (sort off).
Aren't reef tanks fun! Get on thing going, and another pops up. It's like "Whack-A-Mole."