I am building a similar system with three computer fans, all of them blowing down, the difference is that my tank doesn't have cover.
One of the fans I got has an aluminium heat sink, something like this:
I put the heat sink on top of the fan and then put ice cubes or some other cold thing on top of it. The idea is that the metal is cooled by the ice, and the cold air that forms around it is draged by the fan into the tank, following the same principle as an air conditioner. But this is still in testing fase...
Another idea I had, was to put a sealed bag with ice on top of the air pump, or jacuzzi engine, as we call it. This will cool the air that is sent to the aquarium, cooling the water around. The two things I'm not sure about this is if the engine won't be damaged, and if the animals won't have some temperature shock when they go for their beauty bubble bath...