Figure 2 in the article linked below shows the approximate relationship between alkalinity and pH for different CO2 levels. The middle curve represents normal CO2 levels (at least when I made the graph, CO2 has risen outside since then), as one would get by equilibrating seawater with outside air.
if you start at high pH for the given alkalinity (like the blue curve), then aeration with outside or inside air will drive pH down and move horizontal along the graph to the black curve.
In your specific question (dKH of 9 = 3.2 meq/L and pH 8.3) you are just a tad to the left of the curve, and aeration with outside air tends to raise pH (though inside air with higher CO2 will often lower it). If that same 9 dKH has a pH of 8.4, then you are to the right of the black curve, and aeration will lower the pH.
The OP has a pH peak of 8.48. Aeration with inside or outside air will lower high pH unless his alkalinity is above about 4.5 meq/L = 12.6 dKH. He has a nightly minimum of pH 8.15. Aeration with present day outside air may be pretty neutral to that pH if his alk matches NSW, but may raise it a bit with normal reef tank alk, and aeration with inside air might do either, depending on the CO2 level in the home air.
by Randy Holmes-Farley For many aquarists, pH is not something that they have much experience with aside from their aquarium. For many, pH is almost a black box measurement: something to be considered, but whose physical meaning makes little sense to them. This article will Read more here...
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Figure 2. The relationship between alkalinity and pH in seawater with normal carbon dioxide levels (black), excess carbon dioxide (purple) and deficient carbon dioxide (blue). The green area represents normal seawater.
