Is the tank a marine or fresh water one?

If it is marine, then you will not be able to use copper or aluminium in direct contact with the aquarium water, as it dissolves into the water and will kill the fish. I would be equally careful with freshwater as well.

I believe the commercial units cool a water bath of some sort and then pass the tank water through the cooled water in plastic piping.

This kind of thing i guess. http://www.aqua-medic.de/seawater/en/8/skline/