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Water Quality defines all the conditions in the water, which contribute or compromise fish health. Fish "pee" and "poop" in the water and their waste products are mostly nitrogen.
Nitrogen has to be reduced to harmless compounds by naturally occurring bacteria. This is called the nitrogen CYCLE.

Fish wastes / a.k.a. Ammonia----become ----Nitrites - which become ----
Nitrates ----which become ---Food for water plants, green water and algae.
pH is a measurement of acidity and alkalinity in the water. Ideally, it'd be about 7.4 or higher. What holds the pH in its safe range is none other than the Total, or Carbonate Alkalinity.
If you understand these ideas, you will be well suited to fish keeping.
Among the first things you should do when setting up a Koi pond, or if the fish ever seem 'sick' to you - is a test for Ammonia.
Ammonia is the primary waste product of fish, excreted primarily through the gill tissue, but to a lesser extent via the kidney. Ammonia can also accumulate from the decay of fish tissues, food and other organic debris derived from protein. Ammonia accumulations cause reddening of the skin and disability of the gills by its direct caustic effect on these surfaces. Fish suffering in water with high ammonia accumulations will isolate themselves, lie on the bottom, clamp their fins, secrete excess slime, and are much more susceptible to parasitic and bacterial infection.
Ammonia is a big problem in new systems because the bacteria that would naturally dissolve ammonia are not established. As well, even in established systems, ammonia may accumulate in springtime when the water is cold but fish are eating, because filter bacteria have not emerged usefully from hibernation.
Ammonia is capable of ionization below pH 7.4 and so in its ionized state is less toxic to fish.
Above pH 8.0 most ammonia is NOT ionized, and so becomes more toxic. Care should be taken not to increase the pH of a system if ammonia is present but the need to drop the pH or restrict oxygenation to tanks of fish to keep pH down is an overrated aberration in the literature.
Treatment: Water changes and management of the pH near neutral will go a long way to cutting losses from Ammonias, ancillary, less useful modes of Ammonia management include the use of the various water conditioners* that bind ammonia, and the application of rechargeable Zeolites to the system filter. I am still going to tell you that time and water changes are the two mainstays, however.
Ammo Lock (Aquarium Pharmaceuticals) is the only Ammonia Binder I can recommend because they have done the live tissue cell cultures to prove that their product does what it promises to do, and it is NOT manufactured as a bastardization of an aldehyde molecule.
Water that is warm, high in pH or deprived of oxygen will have an enhanced toxicity when ammonias are accumulating. These are all-important considerations as we try to interpret the varying symptomatology of fish at the same ammonia level, for example, but are affected very differently.
pH is a dynamic water quality parameter, which can change overnight. Especially in crowded systems. The net effect of all biological processes is a tax, or drain on pH causing it to "fall" or crash.
pH is a measurement of the free hydrogen ions in the system. pH is measured on a scale of 1 to 14, but the pH required for life lies between 5.5 and 8.5. 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.
pH impacts fish in several ways. 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.
pH contributes to the toxicity of Ammonia. At higher pH values, ammonia is more toxic. Below pH 7.2 most Ammonia is ionized to Ammonium and is far less 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.
pH is prone to fall in un-buffered systems, and can fall precipitously due to Oxygen consumption, accumulation of Carbon dioxide, decay of fish and other wastes, and the normal activity of nitrifying bacteria, which reduce Ammonia to Nitrite.
Crashes from a normal pH all the way down to pH 5.5 can occur overnight. At 5.5 the filter bacteria that may have contributed to the crash will shut down, preventing the crash from dropping yet further. In systems where the pH has been chemically stabilized by Baking Soda, Oyster Shell or any of the commercial buffers, the pH crash phenomena is not commonly seen