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Nitrogen enters the aquaponic system as crude protein in the fish feed. Approximately 30% of protein in the fish food is retained by the fish. Seventy percent is digested and released as solid waste or excreted as ammonia via the gills or as urea (Timmons and Ebeling 2013). Total ammonia nitrogen (TAN) is comprised of two forms that exist in a ratio of un-ionized ammonia (NH~3~, which is toxic to fish) to ionized ammonia (NH~4~+ which in non-toxic). The presence of one form over the other is dependent on pH and temperature. At high pH (basic) and temperature, there is a higher proportion of toxic ammonia. At low pH (acidic) and temperature, ammonia binds to excess H^+^ ions and becomes the less toxic form, ammonium. Generally, water quality tests will give the TAN value, which encompasses both NH~3~ and NH~4~+. The exact value of toxic ammonia can be determined by taking the number that intersects the recorded temperature and pH (Table 7) and multiplying it by the present TAN value (Masser et al. 1999).
Source: (Masser et al. 1999)
Through the process of nitrification, bacteria convert ammonia-nitrogen (NH~3~) to nitrite (NO~2~-) and then to nitrate (NO~3~-). Ammonia and nitrite are 100 times more toxic to fish than nitrate (Somerville et al. 2014). Plants primarily utilize nitrogen in the form of ammonium (NH~4~+), NO~3~- and amino acids such as L-glycine (Rentsch et al. 2007, Sanchez and Doerge 1999). In a fully functioning aquaponic system, ammonia and nitrite values should be close to zero and nitrate should be below 150mg/L. While fish can tolerate much higher levels, upward to 400 mg/L (Timmons and Ebeling 2013), values exceeding 250 mg/L can have negative impacts on plants (Rackocy et al. 2006). From a management perspective, it is important to know the tolerance range of fish and plant species to optimize growth conditions. At excessive levels, these toxic compounds can damage fish gills and stunt their growth.
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.