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pH is a measure of acidity or alkalinity. 7 is neutral, above 7 is alkaline, below 7 is acid.  Different fish have different pH preferences.

Fish from the Amazon region generally prefer acid conditions, so most South American fish including most of the tetras, Angel fish and Discus like a pH below 7.  6.5 would be better than 7 for these fish.  Many of the African, Central American, Asian and Australian fish prefer alkaline conditions, so, for example, goldfish would be happy at 7.2.

Some fish can adjust to wide a pH range while others are fairly specific in their preferences

If you are keeping fish with special requirements you should investigate their requirements before you get the fish.  A mixed tank is often best at neutral.

pH can be tested with a simple test kit using an indicator solution such as bromothymol blue.  These generally cost between $10 and $25.  For people with a large number of tanks that they want to test quickly there are meters available. These cost nearer $100. The meters need to be calibrated occasionally.

pH can be adjusted by adding chemicals.  To reduce alkalinity you can add Sodium biphosphate (sold as pH down), or to reduce acidity you can add Sodium bicarbonate (sold as pH up).  Alternatively, if you prefer to use liquids, Liquid pH up is a dilute solution of Sodium hydroxide, while Liquid pH down is a dilute solution of sulphuric acid.  In most situations pH should be adjusted slowly.

pH will change over time and should be retested regularly.

Adding a buffer can make the pH much more stable.  Two commonly sold buffers are "Bull's Eye" and "Proper pH".  These products are moderately expensive, but many people find them useful.
Steve Challis