These may help. The water quality ones explain ammonia and the N matters noted above.

SRAC 450-459; 4500-4599 -- Recirculating Systems
http://srac.tamu.edu/index.cfm?catid=24


SRAC 460-471; 4600-4699 -- Water Quality
http://srac.tamu.edu/index.cfm?catid=25