Methods
Preparation of agar gel pellets
The ingredients, composition and type of the agar gel pellets prepared and used in this study are shown in Table 1. The method of preparing agar gel pellets was similar to those described in Lim et al. (2015). In general, agar gel pellets were prepared by dissolving the agar gel powder (2 % of the freshwater volume) and red food dye (0.1 %) in 10 ml freshwater, boiled the mixture in a microwave oven, poured the boiled mixture into a glass petri dish for hardening, and subsequently cut it into uniform size (approximately 3 mm × 3 mm × 3 mm) of pellet. Three major types of agar gel pellet were prepared in this study, namely the FT (for-training) pellet, the PAG (pure agar gel) pellet, and the TS (test substance) pellet (refer Table 1 for the content). The FT pellet was used to condition the fish to accept agar gel pellet; the PAG pellet (without inclusion of any test substance) was served as the negative control, while the TS pellets contained each test substance, which was betaine, taurine, inosine—INO (nucleoside), inosine-5′-monophosphate disodium—IMP•Na2 or guanosine-5′-monophosphate disodium—GMP•Na2 (nucleotides).