Fish require an optimal pH. Most species need a pH higher than 5.5. pH 6-8 is probably suitable for most fish. Incorrect pH affects metabolism. Substances such as aluminum ions and heavy metals become extra toxic at low pH. Too high a pH is also harmful. For example, there have been reports of fish deaths after emissions of ammonia.
pH relates to all chemical processes and also influence physical performance. Some how if pH is low or high will make fish behavioural especially in juveniles.