Buffering
The carbonate system links CO₂, pH and KH into one equilibrium. Understanding it explains three otherwise unrelated observations: why pH drifts down in an old tank, why pH swings daily in a planted one, and why bottled pH adjusters do not hold.
In one sentence
Buffering is the water's ability to absorb added acid or base without its pH moving much. In fresh water it is almost entirely the carbonate–bicarbonate system, measured as KH.
The equilibrium
Dissolved CO₂ forms carbonic acid, which dissociates into bicarbonate and then carbonate, releasing hydrogen ions at each step. Adding acid pushes the system towards CO₂ and consumes bicarbonate; adding base pushes it the other way. As long as bicarbonate is present, added acid mostly changes the balance of these species rather than the free hydrogen ion concentration — that is the buffer at work.
When the bicarbonate runs out, there is nothing left to absorb the acid and pH falls fast. This is why a pH crash is sudden rather than gradual.