If all water softening systems corroded fixtures after only 6 months no one would buy them. I have to assume there is something unique about your system that is causing the problem. You have a Ph of 6 pre-treatment. What is the Ph after treatment. I would have a lab analysis pre and post treatment to determine how the softener is affecting the water chemistry. I can't imagine how a softener, which simply removes Ca and Mg ions and perhaps raises the K/Na content of the water, would make a substance that etches disparate surfaces like porcelain and chrome. (Unless it were some complex chemical being formed, Lewis acid etc.) Are you sure you're not abrading the surfaces with a cleaner ?