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 litre 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.
KH (Carbonate Hardness or Alkalinity) – the measure of carbonates and bicarbonates in water; the ability to resist changes in pH. Carbonates help stabilize pH in the pond and are replenished through regular partial water measure and treatment. Hobbyists should be cautious when researching fish, they intend to keep, as many fish that have origins in tropical rainforests where water is extremely soft and acidic, are captive bred nowadays in water with much higher alkalinity and pH levels. Attempting to duplicate conditions in the wild may stress captive bred fish.