All probes are built differently and have different voltage at a specific pH value.
If your pH meter only allows pH7.00 calibration, it will assume that each pH1.00 is 59mV.
Better pH meter allow at least 2 point or even 3 point calibration. By knowing the exact mV at pH4,7 and 10. The pH meter can calculate more accurately rather than assume 59mV/pH.
For aquaria use, single point calibration is good enough. You just need a rough measurement, precise measurements are not required.
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