pH (Aquaculture Water)

pH is the negative logarithm of hydrogen-ion activity, 0–14 with 7 neutral, and in aquaculture it is simultaneously a direct welfare parameter (most farmed species tolerate roughly 6.5–9.0) and a master variable controlling the toxicity of other parameters. Its sharpest interaction is with ammonia: the un-ionized, highly toxic NH3 fraction rises from under 1% to over 10% of total ammonia as pH climbs from 7 to 9, so a TAN reading safe at pH 7.5 becomes dangerous at pH 9 at the same concentration; pH also governs biofilter performance, with nitrifying bacteria operating efficiently only within about 6.8–8.5, and hydrogen sulfide toxicity, which increases dramatically as pH drops below 7. In fertilized ponds the daily pH swing — photosynthesis stripping CO2 pushes pH up by day, respiration returns it overnight — of 0.5–1.5 units is normal, but swings above 2 units signal low alkalinity and weak buffering, usually corrected by liming to hold alkalinity above roughly 75 mg/L as CaCO3. Because pH sits upstream of so many chemical equilibria, farms rarely adjust pH directly (direct acid or base dosing risks killing biofilter bacteria); the practical lever is alkalinity management, with pH monitored continuously in RAS and daily in ponds as one of the non-negotiable baseline water-quality logs.