I'm not familiar with your specific situation AA. If this is a tank where you are using pressurized CO2 then I would think the concern about CO2 fluctuation from the addition of fresh tap water that comes immediately from the pipes would not be a concern because you are already regulating the CO2 in a more powerful way.
There are multiple reasons its difficult to advise about the heat/copper problem I believe. The first is that I don't know the makeup of your system. There are some members here who know what to ask better than me: Are the pipes plastic, lead, copper, new, old? What's the method of heating and what are the coils like and made of? What about any holding of water in the system? Secondly, I've heard that copper test kits may not be reliable enough to help diagnose the situation - anyone remember any info on that?
Of course we should all keep this discussion in context. Its a bit of a tempest in a teapot I guess, as the vast majority, including myself currently, are putting tap water straight in, either via hoses or buckets and of course there are plenty of experienced members who size their changes small enough to use no heating and pretty many don't even use dechlor, although again, I don't recommend that to beginners. By the way, for any of you hardcore hobbyists, there's a fresh article that touches on this subject slightly in the latest AGA issue.
~~waterdrop~~