New pool... Rising pH

[FnC80]

So I just posted in introducing myself and a brand new pool owner. My question is a curiously rising then stabilizing pH. Current test results:

FC - 4.5
pH - 7.8
TA - 90
CH - 250
CYA - 50
Borate - 50
Temp - 82

I find testing water chemistry fun, and interesting in keeping track of fluctuations... especially being a new pool. Now I've noticed when I add acid to reduce pH to 7.6 or less, it climbs back to 7.8 within 8-10 hour and won't budge afterwards. My question is the climbing pH due to aeration from spa spillover, or curing plaster, both? I also find that the pH stabilizes when CSI is slightly positive (0.17) currently, and not sure if that plays a role somehow?


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17.7k gal, 98 ft perimeter, IG quartz, 3HP EcoStar VSP, 1.5HP TriStar Dual Speed, 725 sq ft SwimClear Cartridge, Rainbow chlorinator, Paramount clear O3, Pro Logic controller, 500k BTU heater, AquaVac 500, Spillover Spa, 3 sheer waterfalls, 3 bubblers.

 

[JamesW]

Higher TA or lower pH will create more carbon dioxide, which offgasses and raises pH.

By lowering TA or raising pH or both you get less carbon dioxide and less pH rise.

A negative CSI dissolves some plaster, which raises pH.