Temperature, dissolved oxygen (DO), pH, ammonia, nitrite, carbon dioxide, and salinity must all be maintained within species-appropriate ranges. Fish are ectothermic — temperature directly controls metabolic rate, immune function, and disease susceptibility. Each species has optimal ranges, tolerance limits, and critical thresholds.
Fish experience distress at DO below approximately 5 mg/L (species-dependent). Below critical thresholds, fish show respiratory distress — surfacing, rapid opercular movement, and reduced activity. Chronic sub-optimal DO impairs growth, immune function, and disease resistance. Aeration and paddlewheel systems maintain DO in intensive systems.
Ammonia (particularly unionized NH3) is acutely toxic to fish. It causes gill damage, reduced oxygen uptake, neurological dysfunction, and death at high concentrations. In intensive systems, protein feed creates high ammonia loads that require biological filtration (recirculating aquaculture systems) or high water exchange rates.
Many fish pathogens have optimal temperature ranges that overlap with fish production temperatures. Salmon lice reproduction is temperature-dependent. Aeromonas and Vibrio bacteria proliferate at higher temperatures. Climate change is altering temperature profiles in cage and pond aquaculture, increasing disease pressure.
Modern aquaculture uses continuous water quality sensors (DO probes, pH meters, temperature, salinity) with automated alerts for welfare-critical thresholds. Recirculating Aquaculture Systems (RAS) provide precise control but require sophisticated monitoring. Remote sensing and IoT systems allow off-site monitoring and early warning.
High-welfare aquaculture targets DO above 7 mg/L, ammonia below 0.02 mg/L UN-NH3, temperature within ±2°C of optimum, and pH between 6.5-8.5 for most species. Quarterly review of water quality records against welfare benchmarks identifies chronic problems. Veterinary input on water quality interpretation improves welfare outcomes.