Don't know why your PH is increasing, is normally the inverse... PH lower as chlorine get eliminated...
For sure high PH is caused by hard water... Can't tell you more. You can cut your tap water with RODI...
Putting PH down in hard water will only cause swings... It's an acid... I already tried it to lower my PH from 7.5 to 6.5. The only thing it did is stressing the fishs... And the day after, PH increased to 7.2... PH minus is really not a good product... Stick more with cutting your tap with rodi, it lower ph, don't only put RODI, it will lower it too fast, specially if you have some CO2 in water...
What's your usually tank PH and what fishs are you keeping ? As I said, stability is better than ranges... Sure, if you way out of range, some species can suffer... I won't put neons tetra in ph 9 for sure... but will be fine in 8, 8.3... Their range is 6 to 7...
Also, PH tests are not accurate, and the color result can be affected by chemical in the water (chemicals that doesn't affect PH)...
I would just not care about this, or only cut tap with RODI, don't add PH minus, I'm not the only one that will tell you this...