Algae, TA or what?

[chem geek]

This post is the one showing the difference in active chlorine (hypochlorous acid) level as the pH varies with no CYA and with CYA. The main point is that though it varies much more when there is no CYA, the absolute level is much higher with no CYA so it all depends what you are comparing against. If you compare no CYA against no CYA, then the pH is a big factor.

Even so, if you shock with a lot of chlorine, especially 15 ppm or more, then it's a good idea to lower the pH to 7.2 (or even lower if you are shocking to even higher levels) because the addition of chlorine will raise the pH substantially, even beyond 8.0. Yes, when the chlorine gets used up the pH will mostly come back down but shocking is a sustained high level of chlorine so would be a sustained high pH if you don't lower the pH first.

For a shock of 10 ppm, adjusting the pH isn't that important if it's starting out around 7.5 or so.

As for TA, there was a forum user who found that his pH would rise over time a lot but that when the TA went down to 50 or 40 ppm the pH became stable. It was an extreme case, but we see this a lot in spas (at poolspaforum.com) due to the higher aeration (assuming they are using bleach and not Dichlor, except initially). There are many SWG pools that have lower pH rise at lower TA levels as well. However, in most pools 70 or 80 ppm is OK. A lower TA should be perfectly fine IF one adjusts their target TA and Calcium Hardness (CH) to keep the saturation index near zero. No one is saying to ignore the saturation index if one has a plaster (or fiberglass) pool.