Alkalinity and Hardness — Where to Look When pH Swings

pH swings widely through the day. Ammonia will not come down however much probiotic you add. Shells do not harden properly after a moult. These look like three separate problems, but they often point at the same place — alkalinity and hardness. ■ Alkalinity is what holds pH in place Alkalinity is how well the water resists a shift toward acid. That is what buffering capacity means. With enough alkalinity, pH stays put whether plankton photosynthesise by day or respire by night. Without it, the same conditions push pH up and down across the day. So when the swing grows, looking only at pH is the wrong order. Measure alkalinity first. ■ The target — 100 to 150 mg/L For white shrimp, manage alkalinity between 100 and 150 mg/L. Inside that band the daily pH swing stays under 0.5. Below it, pH starts to move, and heavy rain or a water exchange moves it further. Measure at least weekly. It does not need to be daily, but the value falls as the cycle proceeds, so you need the trend. ■ Nitrification consumes alkalinity This is the most commonly missed part. Taking ammonia through to nitrate consumes alkalinity — roughly 7.14 g per 1 g of ammonia. So the heavier the nitrogen load, the faster alkalinity falls. Alkalinity dropping late in the cycle is not a fault; it is nitrification doing its job. The problem is what happens when it runs out. Nitrification slows or stops, and then ammonia will not come down. No amount of probiotic helps in that state. ■ Hardness — the material for shells Hardness is the calcium and magnesium dissolved in the water. It is often confused with alkalinity but it is a different thing. Alkalinity resists pH change; hardness builds shells. Shrimp make a new shell at every moult, and they take the material from the water. With low hardness the shell does not harden properly, and some animals start a moult they cannot finish. ■ Calcium to magnesium — 1 : 3 The ratio matters more than the absolute amount. Magnesium at roughly three times calcium is the composition reported to suit white shrimp. Seawater is naturally close to that ratio. The problem cases are farms using groundwater or running low salinity. That water is often short of magnesium relative to calcium, so total hardness can look adequate while moulting still goes badly. If you run low salinity, do not read total hardness alone — measure calcium and magnesium separately. ■ When does it fall It falls as the cycle proceeds, because nitrification keeps spending it. It falls after heavy rain too. Rainwater carries neither alkalinity nor hardness, so it simply dilutes both. A large water exchange does the same, so check the incoming water first. ■ Records matter more than thresholds Write alkalinity down once a week and you will see how fast it falls on your farm. Knowing that rate lets you top it up before it bottoms out. Measuring after an accident and knowing the trend are not the same thing. Shrimp365 logs alkalinity, pH and ammonia together per tank so you can see how the three move against each other. Free to use.