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It is the measurement of acidity or alkalinity of the water. A reading of 7.0 is neutral, a pH that is higher than this is considered to be alkaline. If it is lower than 7.0 the water is acidic. Goldfish prefer a water that is more alkaline. There are many different test kits available at petstores that will test the pH for you. I prefer the easier ones that allow you to add some water to a test tube and drop in the chemical drops. You then allow the water to sit in the tube for a minute or two and shake it really good and you will have a color. See the chart below:
The above chart was taken from a brochure that came in my pH test kit made by Aquarium Pharmaceuticals, Inc.