Intensive shrimp culture in Thailand uses 30–57% protein feed with high feeding rates. Organic nitrogen waste from uneaten feed and shrimp excretions decomposes into toxic inorganic nitrogen compounds, including ammonia (NH3 or NH4) and nitrite (NO2). With aerobic conditions, ammonia and nitrite are converted into relatively nontoxic nitrate (NO3), but high nitrate concentrations can stress shrimp. Water exchange is therefore still recommended, especially when nitrate is z 50 mg N l 1