In freshwater aquaculture, what is the ideal nitrate level?

• A. <40 mg/l
• B. <20 mg/l
• C. <35 mg/l
• D. none

Answer: (B)

Nitrate management in freshwater culture focuses on keeping chronic nitrogen buildup low because nitrates accumulate from feed and waste after ammonia is processed by beneficial bacteria. Nitrates are not as immediately toxic as ammonia or nitrite, but high levels over time stress fish, reduce feed efficiency and growth, and can weaken the immune system, increasing disease risk. The practical target is to keep nitrate concentration under 20 mg/L (as nitrate). This keeps water quality stable and supports better growth and health, especially in intensive or recirculating systems. If nitrate climbs toward higher levels, actions like more frequent water changes, denitrification, improved feeding practices, or plant-based uptake can help bring it down. Values like 40 mg/L or 35 mg/L are higher thresholds that carry more stress, and stating there is no target would remove a useful management benchmark.