Managing pH — The Daily Swing Matters More Than the Number

You measure pH in the afternoon and get 8.2. Inside the range, so move on? Not quite. If that 8.2 came after 7.6 before dawn and 8.6 at midday, the water moved 1.0 in a day. To shrimp that width matters more than the number itself. ■ The range — 7.5 to 8.5 For white shrimp, manage pH between 7.5 and 8.5. But that range describes a single moment. Measure once a day and you cannot tell how much the water is moving. ■ Keep the daily swing under 0.5 When the gap between the day's high and low passes 0.5, treat it as a problem that has already started. A wide swing usually signals too much phytoplankton. So the swing is both a burden in itself and an indicator pointing at something else. ■ Why it moves — up by day, down by night By day, plankton photosynthesise and take carbon dioxide out of the water. Less carbon dioxide means higher pH, peaking around 2 to 4 in the afternoon. After sundown photosynthesis stops and only respiration remains. Carbon dioxide builds and pH falls, bottoming out before dawn. The more plankton, the wider the curve. Ponds reading under 25 cm on the Secchi disk swinging hard on pH is no coincidence. ■ Measure twice a day Before dawn, 4 to 6 in the morning, and in the afternoon, 2 to 4. Those are the times that capture the low and the high. Write both down or the width stays invisible. If you already measure dissolved oxygen before dawn, take pH at the same time. One trip, two readings. ■ When pH is high — ammonia gets more toxic At the same total ammonia, a higher pH raises the share of the toxic form. Roughly 1.2% at pH 7.5, about 11% at pH 8.5 — close to a tenfold difference. So a pond running past 8.5 in the afternoon is far more dangerous at the same ammonia reading. That is why you always read pH alongside ammonia. ■ When pH is low — sulfide gets more toxic The direction reverses. For hydrogen sulfide, the toxic share rises as pH falls. Near pH 8 it is under a tenth of total sulfide, and it climbs steeply below that. This is why ponds with sludge on the bottom have accidents after heavy rain. Rainwater pulls pH down, and at that moment the sulfide down there turns more toxic. So pH is dangerous high and dangerous low. Watch both ends. ■ The fix is alkalinity Do not try to move pH directly. The reason it swings is that there is no buffer, and that buffer is alkalinity. Hold alkalinity at 100 to 150 mg/L and pH moves far less at the same plankton density. Alongside that, check clarity for an overbloom and adjust the feed rate. ■ Records matter more than thresholds Write the pre-dawn pH and the afternoon pH side by side on the same date. Within days the widening becomes visible. A 0.3 gap growing to 0.4 and 0.5 means plankton is building, and you can act before it crashes. Shrimp365 logs pH by time of day, draws the daily swing, and alerts you the moment a value leaves its range. Free to use.