After 12 h, triplicate 10-ml water samples were pipetted
from the middle of each aquarium for ammonia-N and urea-N measurements at 0800 h
(end of 12-h dark period). Water samples were again collected at 2000 h (end of 12-h light
period). The maximum accumulation of un-ionized ammonia-N was 4 Ag l1. At the end
of the 24-h experiment, ranges of ammonia-N and urea-N concentrations in water samples
were 0.056–0.478 and 0.0128–0.129 mg l1, respectively. During the excretion experiment,
fish were not fed.