2.1. Fish and experimental design
Fish acclimated to laboratory conditions for 18 months were used in the
experiment. They were kept in pale-blue circular fiberglass tanks and fed twice a
week ad libitum. Sixty fish with mean initial body weight of 116 g were randomly
distributed in six black, white and green cylindrical tanks of 27-l capacity, at a
density of ten individuals per tank (two replicate tanks for each color). The water
used (water exchange rate: 5.9 times per hour) was tap water drawn from the 50-ton
laboratory recirculating system (dechlorinated, filtered and UV-sterilized), provided
with compressed air supply and its physicochemical parameters (means9S.E.M.)
were monitored daily (temperature23.990.07°C, DO6.090.03 ppm, pH
7.2590.005, total ammonia0.2490.006 ppm, NH32.390.1103 ppm,
NO2 - 0.0290.002 ppm, Cl-60.190.53 ppm, hardness11.490.15°d). Constant
photoperiod (8L:16D) was used throughout the experiment, which lasted 14
weeks. Light intensity just over the net cover of the tanks was approximately 75–80
lux. Fish were fed by hand a pelleted diet, once per day (1.5% of initial body
weight), of the following composition: moisture 9.5%, crude protein 47%, crude fat
11.5%, crude fiber 2%, ash 11.5% and N.F.E. 18.5%. Feeding level was not adjusted
during the experiment to prevent possible stress responses caused by the weighing
procedures in fish.