The values of pH were measured using the Hach kit (DREL 2400, Hach Company, Colorado, USA). Water samples were collected from the tanks at Z3, M3, PL1–2 and PL7–8 stages for chemical analyses. Water sample collection and on-site measurements were carried out in daylight, between 10:00 am and 12:00 pm. The concentrations of NH4–N and NO2–N of water in the tanks were measured according to the procedures of Parsons et al. (1984) and Wei (2002)