The reference reading of ORP should be between -190 and 50mV. (to avoid produce the poisonous hydrogen sulfide in over reduction, the ORP value should not be below -190mV.). PH value shows the acid or alkaline state of water (PH value equals 7 is neutral, it is alkaline as PH value is greater than 7, and acid as PH value is less than 7), most of the fish can adapt to life in water around PH=7, but many species have special water preferences.
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