From what I have gathered copper will leak contaminants into the fluid causing a disruption in total dissolved solids. Titanium is used for the larger, more expensive models. This question is aimed towards DIY.
What are the issues with copper? Please elaborate. Many growers use copper. Many home beer brewers use copper. In fact, search with google for the phrase "DIY Wort chiller" for many good ideas that have been posted to the internet. The home-brewers, similar to the koi pond enthusiasts, are our friends.