My wife and I installed a new water furnace with optional desuperheater to heat our domestic water. Since the desuperheater has been running we have noticed blue staining in our tub due to desolved copper. Our local water specialist tested the water and it has a pH of 7.0 on the cold and 7.5 on the hot. The desolved solids were 165 on cold and 200 on the hot. Our whole house is plumbed in pex tubing except for the lines connecting the hot water tank and the geothermal systems.our installer is baffled and does not know what can be done to fix it. Anybody have any ideas?

Look up "cuprosolvency". If your water chemistry is clearly not the suspect, then the easiest thing would be to replace the copper line with something else.

Most manufacturers recommend copper DSH plumbing and against any non-metalic materials due to extreme temperatures. Further, the heat exchanger employed for the DSH is likely copper as well (and going to fail eventually as will the pipe). You could perhaps get an interim heat exchanger of another material, but that would also require a seperate circulator.........
The other thing you could look at would be cathodic protection as is used by DXer's in poorly applied systems.