Why is Ph climbing?

[Rangeball]

If I recall correctly, when my alk was very high as PH rose, alk tended to rise as well. It may very well have been from the process you stated occuring simultaneously, but when my ALK is below 200 it isn't as noticeable.

In any event, simply aerating has never lowered my alk. I had to reduce it with acid first, and try to hold it there as I aerated to bring PH back up.

[chem geek]

In any event, simply aerating has never lowered my alk. I had to reduce it with acid first, and try to hold it there as I aerated to bring PH back up.

My point exactly. Simply aerating just increases the pH (ignoring the increase in alkalinity you saw at high alkalinity levels). It is the adding of acid that lowers the alkalinity (and also lowers the pH). The reason you add the acid first is that the outgassing of CO2 is more rapid at lower pH so by adding acid first you make the process more efficient (occur more rapidly). It would still work if you aerated first and then added acid -- it would just take a lot longer.