Have you done a re-test?
If you could reduce GH by putting something safe in the tank lots of people would be doing it and we would all love to know what it was (but speak to a patent lawyer before you go public )
Tested again this morning & same result. It's an odd one. GH 11dh (196.9ppm) in tank and 14dh (250.6ppm) from tap though water company says 17dh (307ppm).
Consistently actually my tap is 14dh/250.6ppm so that might change stock or at least make some more comfortable.
But again this tank hasn't has a large water change since water went in on 16th Sept.