5.2 Material and Method5.2.1 Plant extract and feed preparationFresh specimens of P. amarus (leaves, twigs) and P. guajava (leaves) were gathered from a location near Can Tho city, Vietnam. The plant samples were processed at the College of Natural Sciences, Can Tho University. The plant parts were exposed to sunlight for three days before being dried at 60°C. Ethanolic extract of the plant was produced by placing 100 g of dried powder in 96% ethanol (800 mL) for 24 h. The collected compound was decanted and screened; and excess solvent was evaporated with a rotary evaporator under low pressure (Nhu et al., 2020).The extract was incorporated into the diet of P. hypophthalmus as either 0.2% P. guajava (Pg0.2), 0.5% P. amarus (Pa0.5), or a mixture of Pg0.2 and Pa0.5 (Mix). The selected levels of extracts were based on prior results regarding the effectiveness of these extracts on the immune response and microbial activity of P. hypophthalmus as well as recommended (Nhu et al., 2020). Four regimens with different types of plant materials (Control, Pg0.2, Pa0.5, and Mix) were prepared. Experimental diets were iso-lipidic, iso-proteic, and iso-energetic. The control diet was the basal diet without supplementation of plant extracts. All ingredients were thoroughly blended into a homogeneous mixture and then pelletized, air-dried, ground, and sieved to achieve the desired pellet size (2 mm). The pellets were preserved at -20°C in properly marked polythene bags.5.2.2 Experimental fish acclimation, facilities, and feeding managementP. hypophthalmus fingerlings (12.05±0.01 g/fish) were sourced from a hatchery in Can Tho city and transported to the laboratory in well-oxygenated bags. The fish were acclimated to experimental tanks (2 m3) with well-aerated water and a natural photoperiod. Fish were fed a basal diet twice a day (at 8:00 am and 4:00 pm) until satiation (3-5% of body weight).