We installed a tankless water heater in our attic about three years ago -- mainly to save space. It has worked fine but I was up in the attic recently and noticed that we have some pretty severe corrosion around the valves and plumbing connections beneath the water heater. You can see this on the attached photos. I've done some reading about galvanic corrosion of dissimilar metals on the internet and that appears to be what is going on here. But it seems to be a pretty extreme case compared with other stuff I see out there on the web. In our case the plumber who installed the heater appears to have used flexible copper connections that are in contact with fittings made of other metals. The corrosion even appears to have affected the gas connection which is the yellow flexible plastic tubing in the third photo. Can we simply replace the flexible copper tubing with tubing made of another material and clean up and/or replace any other corroded parts? Or will the corrosion have already spread into the water heater itself after three years? Clearly i need to get a plumber to come out to do these repairs but what should I be looking out for to avoid a repeat of the same problem? By the way, the flexible copper tubing connects to CPVC pipe at the other end, so we do not have dissimilar metals in contact outside the picture. Any advice how to fix this? Thanks!