Best Guess Table

[aquarium]

No, I aerated the water and simultaneously added acid to intentionally push down the alkalinity, which has resulted in the water stabilizing at the lower pH as well.

I'm not sure about the chemistry at the water surface, but all other things being equal, it appears that the recommended alkalinity range of 80-120ppm will yield the recommended pH range of 7.2-7.8 in a fixed relationship. 80ppm yields 7.2 pH, 120ppm yields 7.8 pH, etc. (at least in my pool )

EDIT:

Though an alkalinity of 100 and pH of 7.4-7.5 works fine for my pool, your pool may be very different and need different parameters.

Which is consistent with my experience of the relationship between pH and alkalinity through that range. END EDIT

BTW, I want to run the pool close to 7.2 pH in order to get the most benefit from the bleach. You said:

At a pH of 7.5, FC of 3.0 and CYA of 30.0 (and TDS of 550, Temp of 80ºF), the HOCl is 0.045. At a pH of 7.2, with everything else the same, the HOCl is 0.052 which is not a huge change, but is still about a 15% change.

Is there a reason NOT to run the pool at 7.2 pH?

[chem geek]

No, I aerated the water and simultaneously added acid to intentionally push down the alkalinity, which has resulted in the water stabilizing at the lower pH as well.
:
:
BTW, I want to run the pool close to 7.2 pH in order to get the most benefit from the bleach.
:
Is there a reason NOT to run the pool at 7.2 pH?

OK, at least now I understand what happened. As for running the pool at the lower pH to get more chlorine benefit, that is true, but it's a relatively small benefit compared to what you can simply get by adding more chlorine. Usually you set your pH for other factors including not stinging the eyes, not risking rusting of metals, etc.

I speculate that the biggest factor in the lower pH will be a tendency to outgas more CO2, even though you've lowered your alkalinity. A pH of 7.2 combined with an alkalinity of 80 (assuming CYA is 30, though this isn't a huge factor) produces exactly the same rate of outgassing as a pH of 7.4 combined with an alkalinity of 120 which is the same as pH 7.5 with alk. 150. You can wander over to the Pool Water Chemistry thread in The China Shop to see a graph of this (it's the last graph in the first post in that thread). I don't want to post it here for risk of scaring people into banning me from ever posting outside The China Shop again

Richard