pH values below 4 and above 10 may be lethal. The best range is between 7 and 9.

With ponds constructed in or near acid-sulfate soil, pH monitoring of pond water is necessary to assure that excessively acidic conditions do not develop. Weekly pH measurements of surface water will provide sufficient information. These measurements are of little value where acid-sulfate soils do not occur. Brackish water normally has a high alkalinity, pH near 8, and is well-buffered against drastic pH changes. Nevertheless, in ponds with heavy plankton blooms, pH can fluctuate between 7.5 and 9.5 over a 24-hour period, with lowest pH occurring near dawn and the highest pH occurring in the afternoon. Also, pH tends to decrease with water depth in all but well-mixed ponds. These dial and vertical pH patterns result from the influence of light on photosynthesis and from the effects of community respiration (Boyd & Fast, 1992).

Plants remove carbon dioxide from water during photosynthesis and cause pH to rise. At night, carbon dioxide from respiratory processes accumulates in the water and the pH falls. Normal dial pH fluctuation generally has no detectable influence on shrimp. With intensive ponds where total ammonia concentrations are high, however, the proportion of un-ionized ammonia (the toxic form) increases with increasing pH and can potentially create problems. Nevertheless, daily monitoring of pH is not necessary (Boyd & Fast, 1992).

Measurements of pond water pH can be made with standard laboratory pH meters, portable pH meters, colorimetric water analysis kits, or universal pH paper. A pH meter gives greatest accuracy. Colorimetric analysis kits typically give pH readings that are 0.5 to 1.0 pH units too high. Universal pH paper is reliable only for determining if a water is acid, neutral or basic (Boyd & Fast, 1992).

With ponds in or near acid-sulfate soils, the best time of day to measure pH is at dawn. At this time, pH will be at its lowest point and low pH problems are most easily identified. With intensive culture, the best time to measure pH is in the early afternoon, because problems are associated with excessively high pH values and pH is normally at its highest by early afternoon (Boyd & Fast, 1992).