If pH keeps falling then your naturally occurring aeration (i.e. not enforcing any more aeration than in normal use) is not (over-) compensating processes in your pool that reduce the pH. I am wondering if a slightly higher TA would increase CO2 outgassing to a level where it compensates your pH drop even without enforcing more aeration. But it's probably enough to just run jets for a while each week or run your pump a bit faster or buy some pool toys that will encourage more splashing to happen.
What is your CH? You certainly don't want to increase TA if you have high CH. And I think one of the processes that decrease pH, is calcium leaving your water and creating scaling somewhere. At 90°F water temperature and CH=1000, your CSI would certainly be in the scaling range at the upper end of the recommended pH range, and still slightly positive in the low 7s. In this case, the equilibrium of your water regarding scaling/corrosion (i.e. CSI=0) would be around pH=7. But I guess in NH you get enough rain that your CH shouldn't go up to these levels.
Usually your pool wants to tell you something by the equilibrium it's trending for. If pH trends are upwards, then that's usually easily explained by aeration. But I am not sure which processes there are, apart from the one I mentioned, that would result in an equilibrium with a low pH.
Not sure what the experts think about that.
