An aquarium without fish was used to measure the background level of ammonia-N
and urea-N in the water (negative control); both were found to be below detectable
levels. Another aquarium without fish was spiked with ammonium chloride (1 mg
ammonia-N l1) and then sampled to determine the amount of ammonia-N lost
(positive control); there was no significant change after 24 h. Positive and negative
ammonia controls indicated that the transfer of ammonia-N and urea-N between the
atmosphere and water was not a significant factor affecting the results of the excretion
experiments..