Safe pH ranges
Every aquaculture species has a working pH range where growth, immunity and moulting all function normally. For most warm-water species that range is 7.5–8.5, but the swing matters as much as the absolute value.
Why pH swings through the day
Daytime photosynthesis pulls CO₂ out of the water, driving pH upward. Nighttime respiration puts CO₂ back in, pushing pH down. A dense bloom in a poorly buffered pond can swing pH from 7.0 at dawn to 9.5 in the afternoon — enough to stress every species in the pond.
Alkalinity as a buffer
Alkalinity — the bicarbonate + carbonate content of the water — resists pH change. Target 80–150 mg/L CaCO₃ for grow-out ponds. Below 40 mg/L, expect wild swings; above 200 mg/L, expect calcium precipitation problems.
Liming strategy and dosing
Agricultural lime (CaCO₃) raises alkalinity slowly and safely — use it for maintenance. Dolomite (CaMg(CO₃)₂) adds magnesium, useful for low-salinity ponds. Hydrated lime (Ca(OH)₂) acts fast but overshoots easily — reserve for emergencies.
Measurement routine
Test pH twice daily (AM before sunrise, PM around 15:00) to capture the full swing. Alkalinity: once a week. Log both — pH alone hides the alkalinity story.
Troubleshooting persistent lows / highs
Persistent low pH: acidic soils leaching into pond water, or organic overload from uneaten feed. Lime + siphon accumulated sludge.
Persistent high pH: dense bloom + low alkalinity + low DIC. Partial water exchange dilutes the bloom; molasses adds carbon to feed heterotrophs.