It is a confusing one. Alkalinity is the waters buffering capacity i.e it’s ability to neutralise hydrogen ions and resist pH change.
There are many ions that contribute to total alkalinity. Phosphates, nitrates and silicates etc but the main contributor to alkalinity is carbonates.
In waste water treatment it is said that it takes 7.14ppm of alkalinity as calcium carbonate to nitrify 1ppm of ammonia. Thus as the tank ages, the buffering capacity is used up, hydrogen ions are no longer neutralised and the carbonate equilibrium falls in favour of the hydrogen ion and pH declines. Because there is no alkalinity and thus (in theory) no nitrification ammonium goes up. This is the non toxic state of ammonia. Its is kept non toxic because if the low pH. If you than decide to do a water change or add a carbonate buffer the buffering capacity rises and hydrogen ions are neutralised. The carbonate equilibrium shifts and pH rises. A build up of ammonium now becomes toxic free ammonia and causes acute toxicity and death. This is where the term ‘old tank syndrome’ comes from.
As new studies and information emerges it is becoming clear that there are many microbes that can contribute to nitrification that differ between waste water and aquarium filters who’s growth are dictated by many factors. This throws the whole 7.14ppm alkalinity per 1ppm ammonia statement with respect to aquariums in to question.
Carbonate hardness KH
is the carbonate part of alkalinity.
The API KH
test kit actually measures alkalinity.