I've had this issue ever since I moved into a house with a pool 5 years ago. The plaster was redone a couple years before we moved in, which we hoped was causing the issue, but now that it's been at least 6 or 7 years since the re-plaster job, I think it's safe to say that we're well beyond the point that this should still be an issue. After some years of posting, researching and trying various suggestions, I finally just gave up. I have no water features, and I've tried everything suggested here, including lowering TA all the way down to 50, but nothing has stopped -- or even noticeably slowed down -- the pH rise. I've also had local pool professionals take a look at my situation, but they seem to know far less than the people on this forum.
For a while I theorized that perhaps my cleaner setup was resulting in very low pressure on the suction side of the pump, causing cavitation and resulting in excessive aeration which, in turn, might cause pH rise. I later replaced my cleaner line with a much larger diameter pipe, however, reducing the negative pressure a bit on the suction side of the pump and reducing cavitation, but my pH rise was not effected.
On many occasions I've considered adding borates to slow the rise down, and may still do so someday, but given that this doesn't actually reduce the amount of acid needed, I just never pulled the trigger on it. According to the information I've read, adding borates slows the pH rise, but it also requires equivalently more acid to raise the pH by the same amount. My pool seems to be relatively stable other than this, so I decided not to mess with it by adding other chemicals.
In the end I just came to the conclusion that my regimen requires me to add two liquid chemicals instead of one. Over time, and after a lot of trial and error, I settled in on a range of Cl and pH that would allow me to add the two chemicals at the same time (i.e. one after the other) about twice a week during the summer, and maintain CSI balance according to the pool calculator. When I need to add chlorine, I simply need to add acid as well. The numbers and ranges shift during the winter when the water temp drops, which effects the CSI balance, but with far fewer chlorine additions needed, it's not all that big of a deal to add Cl and acid on different days if needed.
I'm still endlessly frustrated to hear about other people not having to deal with this (it's just not fair!), or to read about such simple solutions solving the issue for others when these methods seem to have zero effect for me. I'm also still really curious why my direct neighbor does not have any such issue with his pool (I did some measurements over a number of days between their chemical additions and their pH did not budge.) In the end, however, I had to try to come to terms with the fact that there are unknown factors involved, and that some pools, for whatever reason, are just going to have this issue. I can definitely find other people complaining about this, so even though the people with this issue are in the minority, they're not alone.
In my case, my best guess at this point is that perhaps there is something about my plaster that's causing this -- some specific issue with the composition -- that will hopefully be resolved when it's eventually time to redo the plaster once again. I'm still years away from that, however, so I just live with the idea that some pools have this issue.
Given that my pool chemistry is fairly trouble free other than this issue, I think that things could be a lot worse. The need to add acid as well as chlorine all the time is annoying, but relatively speaking it's not "that" big of a deal.
Please let us know if you ever find a solution -- I'm always eager to hear what other people do to combat this.
Thanks,
Larry
