I saw the link for the what you call the cooling pad. It looks like nothing more than a filter/pad for a swamp cooler. The pad/filter does not cool the the water, it actually heat's it up. If you run it through your coils before it gets to the top of the pad and drips down through the filter you may have a chance. But I'm not sure how much volume it can do. At 20C, 68F you will need a large set of coils as well as volume of liquid going through it to even help. Keep in mind, if it's in an enclosed system the coils will heat it up as well, further reducing it's effectiveness.
I'm not sure where you have your system but I would use a geothermal heat exchange system, if I can at all run the pipe/tubing outside. Either as a closed loop, or open loop system. Although I think a open loop would be the most cost effective even in the long run.
As for aluminum I am not sure if it reacts any different to the
nutrients than copper. Stainless steel is the only metal that I would be sure of. I don't remember what department it was in but they do have them already formed in coils, they were just too expensive for me to consider.