What Is normal pH?
There is no “normal” pH that applies to all fish. Because fish originate in ponds, rivers, streams, lakes, and oceans that have different pH levels, their needs are different. Saltwater fish prefer an alkaline pH of 8.0 or above. Freshwater fish thrive in a range lower than that, somewhere between 5.5 and 7.5, depending on the specific species.
Keep in mind that pH is not static, it changes over time; in fact, it even changes over the course of a single day. Typically it drops at night and rises during the daytime. The pH will change as new fish are added or removed, as water is added or changed, and as the biological processes change in the tank.