8.59 is acceptable for pH, and there's a reasonable chance that the measurement is incorrect. If the pH actually is high, the tank might just need a bit more aeration. I might go for a bit more rippling on the water surface, if that was easy to do. Does the tank have a skimmer?
The 8.6 at night is unlikely to be accurate unless there's a lot of Kalk going into the system.
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Jonathan Bertoni
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