I think that the value is not accurate. It was initially displayed as PH 4 or less (weakly acidic) in the aquarium, and I thought that there was no problem with water quality. After purchasing the PH meter, I measured it, it was PH 7.5. When measured with tap water, which was correct, the litmus paper was PH.6.4 and the pH meter was PH 7.1. I think the PH meter is correct.