Especially in crowded systems. Individual species will have varying demands as far as pH. Ignorance of the requirements of each species will result in the death of the animal in question. First, if the pH is too low, a condition called Acidosis results. Symptoms are anorexia, and then production of excess slime, isolation, and resting on the bottom, finally, streaking of the fins, and death will occur. If the pH is too high, the fish will produce excess slime, and will gasp at the surface.
Losses can be major.
Alkalosis is hard to reverse once it occurs. On the other hand, Acidosisis is rapidly corrected once the pH is brought up to a suitable range. At higher pH values, ammonia is more toxic. This has relevance if you are considering raising the pH in a system with accumulating ammonias. There is a routine, inexpensive test that measures pH, and compares the result to a color chart for the diagnosis.