Alkalinity, often referred to as "carbonate hardness," or German carbonate hardness, is the measure of carbonate and bicarbonate concentrations in your aquarium water. Alkalinity is a measure of the ability of a solution to neutralize acid without changing the pH. It both controls and maintains water pH. Carbonate hardness is measured in degrees (dKH), parts per million of calcium carbonate (ppm CaCo3), or milligrams per liter (mg/L).
Alkalinity is not the same as pH because water does not have to be strongly basic (high pH) to have high alkalinity. Alkalinity is related to the amount of dissolved calcium, magnesium, and other compounds in the water and as such, alkalinity tends to be higher in "harder" water. Alkalinity is naturally decreased over time through bacterial action which produces acidic compounds that combine with and reduce the alkalinity components.
In an established pond, the ideal Alkalinity measurement should be around 100 ppm. Readings from 50 to 200 are acceptable.
High alkalinity is normally prevented by routine water change outs assuming the water being replenished has a lower alkalinity than the pond water.
Ponds with vinyl liners or of fiber glass construction tend to show a decrease in alkalinity over time and may need supplements to maintain an acceptable level. Raise alkalinity by adding Calcium Carbonate, concrete blocks, oyster shells, limestone, or even egg shells.
Established ponds will normally maintain their equilibrium pH value if sludge and decaying organic material is routinely removed from the pond, mechanical filter, and biological converter. Scheduled water change outs (10% per week for a small pond, less for larger ponds) are also helpful.