Below was extracted from here.
Alkalinity and hardness are both important components of water quality. However, these two aspects of water chemistry are commonly confused. The confusion relates to the term used to report these measures, ppm CaCO 3 (same as mg/L).
- Total alkalinity indicates the quantity of base present in water –bicarbonates, carbonates, phosphates, hydroxides, etc.
- Hardness represents the overall concentration of divalent salts (calcium, magnesium, iron, etc.) but does not identify which of these elements is/are the source of hardness.
It is important to recognize the difference between hardness and total alkalinity when farming aquatic animals. The determination of whether water is acid, neutral or base is defined by pH. However, alkalinity measures the total amount of base present and indicates a pond’s ability to resist large pH changes, or the “buffering capacity.” The most important components of alkalinity are carbonates and bicarbonates. The total alkalinity concentration should be no lower than 20 mg/L CaCO 3 in production ponds. Pond pH can swing widely during the day, measuring from 6 to 10, when alkalinity concentrations are below this level (Fig. 1).
Large daily changes in pH can cause stress, poor growth and even death of the farmed animals. Most aquatic organisms can Fig. 1. Changes in pH during a 24-hour period in waters of high and low total alkalinities (Wurts and Durborow, 1992).
If alkalinity and hardness concentrations are below the suggested level, both can be increased by adding agricultural limestone [CaCO3 and CaMg(CO3)2]. Agricultural limestone will not increase pH beyond a maximum of 8.3. The use of hydrated lime (CaOH2) or quick lime (CaO) is not recommended because either of these compounds can cause the pH to rise very rapidly, to levels that are harmful to aquatic life.
Extracts From: Liming Acidified Lakes and Ponds by Virginia Tech
Test the pH at different times of the day over several weeks to reliably determine the range of pH values. Generally, if the lake is an acidic or soft-water system with pH values less than 6.5 (and acid neutralizing capacity less than 50 microequivalents per liter), total alkalinity levels less than 20 milligrams per liter (mg/L), and total hardness concentrations below 25 mg/L, liming may be beneficial.
Reliming should occur whenever pH values drop below 6.5, total alkalinities drop below 10 mg/L. Maintaining the pH near neutral (pH 7) and total alkalinities > 20 mg/L will minimize stress to fish and prevent the bioaccumulation of toxic metals, such as mercury, in fish tissue.