Water Quality Management in Commercial Aquaculture

Water quality is treated in many project plans as an operational discipline. In reality, it is a design constraint. Every water quality parameter — dissolved oxygen, TAN, nitrite, nitrate, CO2, pH, alkalinity, temperature, salinity — is a boundary condition that the facility must hold across the full biomass curve. Facilities that treat water quality as something to manage after commissioning rather than something to engineer for at design invariably fall out of specification during peak load.

Alkalinity and pH

Nitrification consumes alkalinity — approximately 7.14 g of alkalinity (as CaCO3) per gram of TAN oxidised. Without automated bicarbonate or sodium hydroxide dosing, pH drifts down, nitrification slows, TAN accumulates and the biofilter enters a downward spiral. Alkalinity control is a design decision: dosing capacity, storage volume, redundancy and control logic must all be specified before the biofilter is ordered.

TAN and nitrite

TAN is generated by protein metabolism and oxidised by ammonia-oxidising bacteria to nitrite, then by nitrite-oxidising bacteria to nitrate. Nitrite accumulation — the classic 'brown blood' failure — occurs when the second step lags the first, typically during biofilter maturation, temperature swings or after chemical shocks. Salinity or chloride dosing can buffer nitrite toxicity but does not remove the underlying imbalance.

Carbon dioxide and dissolved oxygen

Dissolved oxygen and CO2 are inversely coupled: as fish and biofilters consume oxygen, they produce CO2. Degassing must be sized to hold CO2 below species-specific thresholds at peak load. High CO2 depresses pH and reduces oxygen affinity in the blood, compounding the effect on growth and survival.

Temperature and salinity

Temperature drives metabolic rate, oxygen demand, nitrification rate and pathogen dynamics. Temperature control is an energy decision as much as a biological one. Salinity affects osmotic load, ion balance and nitrite toxicity. Both should be treated as fixed design parameters, not as variables to be adjusted during operation.

Monitoring, alarming and response

Monitoring is only useful if it triggers action. Alarming thresholds, escalation paths and response protocols must be documented and tested before stocking. Lenders and insurers increasingly ask to see the operational readiness package — monitoring, alarming, redundancy — as part of technical due diligence.

Continue in the RAS planning cluster: Dissolved Oxygen Management for Bankable Aquaculture Farms and Biosecurity Zoning and HACCP for Commercial Aquaculture. Pillar guide: Why Successful Aquaculture Projects Begin Long Before Equipment Is Purchased. See the full outline in the RAS Planning Topic Cluster.