I think the underlying issue is that many aquarists, and probably all beginners, do not comprehend one basic fact, and that is the incredibly complex relationship of a fish to its aquatic environment. No land animal, with the possible exception of the amphibians [note, water again], has this close a tie to its environment. The day-to-day operations that occur just to keep the fish alive is completely dependent upon the environment. This includes not just water parameters [though many clearly do not understand the impact of these] but every aspect of the physical environment. Every individual species of freshwater fish has evolved to function in the specific environment in which it finds itself naturally, and while some species can move outside this environment in some respects, or to some extent, none can do so completely without consequences. And the latter is usually death, immediate or slowly.
The correct functioning of the fish's physiology depends upon what it receives or does not receive from the water in which it lives. The fish must adjust the pH of its blood to match the water it lives in, so if this pH should change the fish is immediately required to commence internal processes that will equate the blood pH, and this takes considerable energy, and that means less energy for other functions, such as maintaining its immune system. Laura Muha had a monthly column in
TFH a few years ago, entitled "The Skeptical Aquarist," and she came up with an excellent analogy. Driving a car on a flat road at 50 km/hour takes "x" amount of energy; to maintain that same speed as you climb a hill requires considerably more energy. A fish uses "x" energy to function normally within the preferred environment for which it is designed; as soon as it is forced to move outside that environment--any aspect of it--it has to "switch gears" and go into overdrive or it will not survive. Other processes suffer the consequences, and this weakens the fish even further.
Stress is certainly a major factor; it is directly responsible for about 90% of aquarium fish disease. How stress affects fish, and the degree to which is does and is often irreversible, may surprise you. Mike [AbbeysDad] has my article on stress available on his blog, here: