pH is a measurement of the acidity or alkalinity of a substance. It is measured on a scale of 0 to 14, with 0 being the most acidic and 14 being the most alkaline. A pH of 7 means the substance is neutral. The pH scale is logarithmic, meaning the difference between any 2 whole numbers is a multiple of 10. It is important to recognize that small numerical changes represent significant changes in pH. The pH level of your aquarium should be monitored regularly because most fish have a narrow range of pH in which they will thrive.
9.0 (100 times more alkaline than a pH of 7.0)
8.0 (10 times more alkaline than a pH of 7.0)
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6.0 (10 times more acidic than a pH of 7.0)
5.0 (100 times more acidic than a pH of 7.0)