hi, it's aerodynamic. using the principles of aircraft wings. when air passes over the wings. air pressure reduces, air velocity increases, hence the temp. drop.
So, by placing a inverted wing like design combine with evaporation. u can actually reduce 5 degrees C below ambient temp. I have done mine and i managed to bring down from 32 to abt 27. My tank is built into a wall, so temp variation is not high.
hope this helps