Hi Steve,
As Mick confirms the situation in this hobby with pH is very similar to the situation with nitrate. People have a problem in their tank which affects the pH at the same time, i.e, some toxic agent which is killing their fish and so they blame the pH instead of looking deeper into the cause and finding the agent responsible for simultaneously killing the fish and for affecting the pH.
Think about what happens for example if a moderate sized fish dies and goes unnoticed in the tank. The body decays and pollutes the tank. The ammonia in the tank raises the alkalinity but it also poisons the other fish in the tank. So the hobbyists measures the pH and discovers that it is high...and concludes that high pH kills the fish.
The reverse reaction happens more often, especially in CO2 injected tanks. If you overdose your CO2 application it drives the pH low due to the carbonic acid right? If the alkalinity of the water is low (i.e, low KH) then the acidity of the water becomes extreme. The fish die. What killed the fish, low pH? No, they died of CO2 poisoning which causes the blood to acidify. The pH of the water is irrelevant at that point.
So when we have an unusual pH reading and when we suspect a problem we need to troubleshoot the cause of the unusual pH, because that will be the same cause that is causing the toxicity. Fish have a very wide tolerance for the variations in natural pH, but the pH must be considered within the context of the system. South American fish especially live in highly acidic waters which are often stained by high concentrations of tannic acids, so it's very unlikely that a low pH can kill these fish. However, many people dump highly toxic acids into the tank trying to control the pH, strong acids such as Phosphoric or Sulfuric acids which are strong acids and which burn the gills and mucous membranes resulting in asphyxiation. These strong acids also drive the pH low but they are toxic whereas weak, non-toxic acids such as tannic or carbonic acids are weak acids but which drive the pH just as low without doing any damage.
Cheers,