Adding this for folks that look up 'Alkalinity' on the forum and find this thread - as well as TP.
Alkalinity – excerpt from “Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds” by William A. Wurts and Robert M. Durborow, Southern Regional Aquaculture Center 1992
Alkalinity
The quantity of base present in water defines what is known as total alkalinity. Common bases found in fish ponds include carbonates, bicarbonates, hydroxides, phosphates and berates. Carbonates and bicarbonates are the most common and most important components of alkalinity. Alkalinity is measured by the amount of acid (hydrogen ion) water can absorb (buffer) before achieving a designated pH. Total alkalinity is expressed as milligrams per liter or parts per million calcium carbonate (mg/L or ppm CaCO3 ). A total alkalinity of 20 mg/L or more is necessary for good pond productivity. A desirable range of total alkalinity for fish culture is between 75 and 200 mg/L CaCO3 . Carbonate-bicarbonate alkalinity (and hardness) in surface and well waters is produced primarily through the interactions of CO2 , water and limestone. Rainwater is naturally acidic because of exposure to atmospheric carbon dioxide. As rain falls to the earth, each droplet becomes saturated with CO2 ; and pH is lowered. Well water is pumped from large, natural underground reservoirs (aquifers) or small, localized pockets of underground water (groundwater), Typically, underground water has high CO2 concentrations, and low pH and oxygen concentrations. Carbon dioxide is high in underground water because of bacterial processes in the soils and various underground, particulate mineral formations through which water moves. As ground- or rainwaters flow over and percolate through soil and underground rock formations containing calcitic limestone (CaCO3 ) or dolomitic limestone [CaMg(CO3 )2 ], the acidity produced by CO2 will dissolve limestone and form calcium and magnesium bicarbonate salts: CaCO3 + H2O + CO2 = Ca+2 + 2HCO3 - or CaMg(CO3 )2 + 2H2O + 2CO2 = Ca+2 + Mg+2 + 4HCO3 - The resultant water has increased alkalinity, pH and hardness.
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