Aquadream,
what you need to realise is that pH is an arbitrary and pretty meaningless measurement. It is not a measurement of anything chemically tangible, but rather it is a
mathematical procedure, referred to as a "chemical speciation calculation". This is the function used to calculate pH:
pH = -log aH+
aH+ is Hydrogen ion activity. The important thing to note here is that pH does not measure the true concentration of hydrogen. It measures the
effective concentration. This is because it must account for the fact that other ions around the H+ ions affect their ability to participate in chemical reactions, basically changing the hydrogen ion concentration in any process that involves H+.
pH gives a rough overview of
all the chemicals present in a solution that contribute to acidity or alkalinity, it can't tell you anything specific.
An obvious analogy is TDS. TDS doesn't actually
mean anything at all. TDS is (sort of) another chemical speciation calculation, insofar as it takes a reading (conductivity) and applies a mathematical function to it to obtain a different reading.
Now, what does
total dissolved solids actually tell us? Nothing. It tells us roughly how much "solid" is dissolved in the water. But what is that solid? It might consist entirely of potassium bicarbonate, in which case the fish won't care, but it might consist entirely of chloramine and ammonia, in which case the fish will die. So two identical measurements of TDS (for argument's sake let's say 100 and 100) look identical, but can be the difference between life and death. This is because it is an overview of everything in the water.
Back to pH, it is exactly the same story. It gives a rough outline of
everything present in the water. Let's take a theoretical swing of pH from 7.0 to 6.0. Now what has actually changed to cause this reading? The
power of hydrogen is what has changed. What does that even mean, power of hydrogen?
What Clive said here is the crux of the issue:
Carbonic acid is a very weak acid and does not produce enough free H+ to make a difference in the Na+:H+ exchange mechanism.
The fish don't care about the pH changing. They care about whatever happened to cause the pH to change. If the pH change was caused by a significant change in the exchange mechanism, then it changes the chemical capability of the water. The water will behave in a different way in relation to other chemicals.
If the pH change was caused by dissolution of a very weak acid, the chemical composition of the water remains basically the same. The dissolved acid is too weak to change the way in which the water behaves in relation to other chemicals. All the chemical processes/ reactions that happen will be virtually unaffected by the extra acid, and will remain pretty much the same.
I hope I've made this clear as it's a long time since I studied this. If there are any mistakes in what I've said I'm sure Clive will pick up on them.