The experiment was carried out in a closed re-circulation culture
system in a series of 21 composite settlement tanks (3 per treatment)
with a volume of about 500 L per tank. The settlement tanks were
connected to a sedimentation tank which contained sand and stones
(1–2 mm) as a biological filter. Tap freshwater was aerated for 24 h to
allow chlorine to evaporate and fed into each tank at a flow rate of
3 L min−1
Aeration with one air-stone diffuser was provided to
individual tanks via a low pressure electrical blower. In addition, the
walls of tanks in the culture system and the biological filter system
were cleaned by manual scrubbing to remove slime from fish and
microbes twice a week.