Zcomp has great info.
Hardness in tap water is largely composed of Calcium carbonate, and will buffer your pH quite high- HOLD it there, despite efforts to change it.
In your case, I would not even go with lemon juice because you will be fiddling with it for so long. Get a bottle of pH down and a proper liquid indicator whil you're at it if the strips are annoying (thye annoy me kinda lol!) Flush with water at pH 6.0 and that should fairly quickly bring you down to the magic 6.7 point where cannabis is happiest.
Just keep an eye out for herms. You may be fine, or not; no way to tell until you're there!