For the estimation of apparent digestibility coefficient (ADC) of the experimental diets, chromic oxide (Cr2O3) (Sigma-Aldrich, St. Louis, USA) was included in the diets as an inert indicator at a concentration of 1.0%. The digestibility trial was conducted in fiberglass fecal collection tanks of 300 L capacity designed according to Cho et al. (1982). New sets
of red sea bream and olive flounder with mean body weight of ~50 g were stocked into each tank at a density of 40 fish per tank and each group of fish was fed one of the test diets. The tanks were supplied with cartridge-filtered seawater at a flow rate of 1 L min−1 and aeration
to maintain enough dissolved oxygen. The digestibility trial consisted of three periods of 10 days. In each 10-day period, the fish were allowed to become acclimatized to the feed for the first three days and feces were collected over the next 7 days. Then, diets were randomly changed between tanks and the procedure was repeated for two more times giving a total of three fecal samples for each diet. All feces collected from each tank in each period were pooled and frozen at −20 °C for analysis. After each feeding, the tanks and the settling columns were
thoroughly cleaned to eliminate all feed wastes and fecal residues. Chromium oxide content of diet and feces samples was analyzed by the method described by Divakaran et al. (2002). The apparent digestibility coefficient of the experimental diets was calculated through the following
formula: