If pH changes more than 0.3 units per day, it can send the fish into pH shock. This means one must keep the pH of their tank constant and stable. Second, fish have adapted to surviving in a certain pH range. Don t worry, though. Generally, most fish can adjust to a pH that is outside their optimal range.
Freshwater fish are very adaptable as a rule.
Many of them are from environments in which the pH and hardness of the water fluctuate from time to time so they have to adapt in order to survive those changes. Remember, the time to make major changes in the pH of the tank water is before one adds the fish. This filtration is necessary to the processing of fish wastes and lowers pH by releasing carbon dioxide into the aquarium s water supply. It s a gradual process, but in a heavily stocked tank, the pH may drop by a whole point, from 7.6 to 6.6, in a period of a few months. The drop is gradual, so the fish usually survive, but weekly pH tests with the IQ120 pH meter are recommended just in case.