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From Aquarium Pharmaceuticals -- pH readings fluctuate during the day so pH testing should be done at several different times. Pond fish prefer a pH range of 6.8 to 8.0. Less than 6.8 means the water is too acidic / More than 8.0 means the water is too alkaline. A pH crash can kill every fish in the pond. Prevent fluctuations in pH by using a pH stabilizer.