Fries of red tilapia, Oreochromis niloticus x Oreochromis mossambicus were obtained from Hatchery Unit of El-Max Research Station, Alexandria (NIOF) as natural hatchery during summer 2010. Fish were transported in oxygenated cellophane bags to the laboratory. Fishes were randomly distributed in 12 glass aquaria of 33-litre size. Each aquarium contained 10 fishes.
The fish biomass in the aquaria was about 2.5 g/aquarium. Fishes of each aquarium were group-weigh to avoid handling stress to determine their average weight. Each aquarium was equipped with a biological filter containing high porosity filter sponge which was washed thoroughly every two-days and cleaned regularly.
Aquarium water was oxygenated through the biological filter. Fishes were kept for one week to acclimate to the laboratory conditions. Ten fishes with more or less similar initial weights (206±11 mg /aquarium) were selected and randomly allotted to each experimental aquarium.