In a recirculating aquaculture system, what TAN level is acceptable?

• A. 0 ppm
• B. 25 ppm
• C. 0.5 ppt
• D. 2 ppt

Answer: (D)

Total Ammonia Nitrogen (TAN) must be kept under tight control in a recirculating system because ammonia is toxic to fish and increases as pH and temperature rise. In a well-managed RAS, the biofilter and continuous water management work to keep ammonia from accumulating, but occasional spikes can occur and the system should tolerate them up to a limit.

Choosing the 2 ppt level as acceptable reflects a training idea that a properly operated system with good filtration and dilution can handle brief TAN elevations up to that threshold without immediate harm. It emphasizes the need for ongoing monitoring and biofiltration to prevent chronic exposure, while recognizing that the exact safe level depends on species, temperature, and pH. In practice, the goal remains to keep TAN as low as possible.

The other options represent impractically low or clearly unsafe levels for most recirculating systems, reinforcing that zero is ideal but not realistically maintainable, and very high levels would be harmful.