Why does your pool's PH keep increasing?

Gunite and plaster have a high demand for acid because of the lime in the cement. And or high calcium make up water.

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Your pool will naturally increase in pH due to swimmer's waste, sweat, urine, etc...

Keep your pH at 7.6-7.8, not 7.2. 7.2 pH is base upon the Langelier Index which was used for local water companies, but there's a world of difference between pools and how the water company treats your tap water. Good index but bad application for pools.