Fish in three aquaria were assigned to each of the experimental
diets and fed to satiation twice daily. Bi-weekly feed consumption was recorded. Total
weight and number of fish in each aquarium were determined every 2 weeks. From
week five to 10, the aquaria were cleaned 1 h after the last feeding to remove remaining
diet or feces every day. The feces collector designed by the Tokyo University of
Fisheries Ogino et al., 1973 was installed instead of the normal siphoning column. Each morning, the feces collector was released for feces collections. The feces were
separated from the water and stored in a freezer before freeze-drying. The experimental
feed and pooled feces samples were analyzed for protein AOAC, 1980 and chromium.
Furukawa and Tsukahara, 1966 . The apparent digestibility of dietary protein was
calculated according to Cho et al. 1985 . At the end of the experiment, three fish from
each aquarium were anesthetized with 100 mgrl of tricane methane sulfonate MS-222
Tissues from internal organs such as the liver, kidney, anterior and posterior intestine
were dissected and fixed in Bouin’s fixative for 24 h. They were processed using
standard histological methods Humason, 1972 . Samples were processed through an
alcohol series and embedded in paraplast. Sections of 3–4 mm in thickness were cut and
stained with hematoxylin and eosin. The sections were observed under a light microscope. Three sections of each organ per fish were examined for any pathological
changes.