http://www.procooling.com/articles/h...r_chille.shtml
That was my first stab at H2O chlling so do not take it as discouraging. I learned a lot from it. Basically, you need to have the water recirculate through the chiller (which you will because it is an aquarium). Also, insulation is key. You must insulate the cold side of the block. Otherwise, most of the cold goes into the air, not the water. I do think you can be successful using a cooling block like I did though. Because there are no true heat sources in the aquarium, you can just recirculate the water through the chiller and expect it to slowly, but surely, chill the water.
-Kev