A 6.9 pH is not low at all. It is virtually neutral. Low is something below 5 or maybe 5.5. A 6 dKH alkalinity reading is a perfectly good reading. Once you start adding baking soda, shooting for some specific KH, you are trapped into monitoring and adjusting one more parameter to accomplish essentially nothing. I used to do that, but tried just ignoring KH and haven't regretted it at all.