Here is some info I have put together over time hope this helps…
Alkalinity is the total carbonate hardness. Or a reading of KH or dKH (degrees carbonate hardness). This is many different minerals that are alkaline. This does not tell you how acidic or alkaline water is because it does not take into account the acidic things that neutralize the alkaline minerals. PH does tell you how acidic or alkaline the water is, but does not let you know why it is acidic or alkaline.
When we use the term pH we are talking about the amount of hydrogen ions, or whether it is acidic, neutral, or basic from 0-14 acidic less than 7 basic more than 7. Older book will say alkaline instead of basic and this is how things get confusing.
The term Alkalinity is used to mean different things depending on how it is defined (It can mean basic or carbonate hardness). When we use Alk we mean the total amount of carbonate hardness. Carbonate hardness is an old term and I should say something like “The buffering substances like Carbonate, Bicarbonate, borate, hydroxide, silicate, and phosphate.” But who has the time for all that. These minerals resist the change in pH. This is why we commonly call them buffers. The proportions and amounts of these minerals affect the pH. In simple terms if you don’t have enough alkaline minerals in the correct proportions meaning the Alk is low, the pH will be low.
This does not mean the pH will be where you want it to be because there are acids that also affect pH. Mainly carbonic acid (waste decomposition) and CO2 (respiration).
There is maximum amount of minerals that will stay dissolved in the water. So the total alk can be too high. If you keep adding buffer the chemistry will get all wacky (it precipitates out) and you will have trouble keeping calcium up. There is not normally too much threat to most live stalk if you keep the salinity and pH correct. Some of the supplements will raise the salinity more than others and I have been surprised at times, so keep watch on it. You want to test for alkalinity (carbonate hardness) because it becomes depleted as it reacts with acids. A buffer has a pK value, this is the range the buffer resist change in pH. This buffer (extra alkaline minerals) as they are depleted nothing happens to the pH and once lost to reacting with acids the pH starts to drop. My alk will go from 10 dKH down to 8 dKH with no change in pH.
The higher alk is important for corals because magnesium, strontium, calcium, and carbonates are the building blocks of coral skeletons. With higher amounts of them, we get good coral growth for longer. So adding buffer replaces the (extra alkaline minerals) that are lost to coral growth, reactions with acids, and maintains pH.
Do you have to add buffer? Not if water changes keeps up with the need to replace…
A good book is Marine Chemistry by CR Brightwell
