Breeders and keepers of Siamese fighting fish (Betta splendens) often add Indian almond (Terminalia catappa) leaf extract to the water in the aquarium, but there is little information available about the effects of extract concentration on water quality and fish performance. In the current study, the effects of T. catappa extract on water quality, growth performance, feed utilization, skin coloration, bubble nest formation, digestive enzyme activity, muscle quality, whole-body composition, and hematological parameters of B. splendens were investigated. Seventy-five juvenile male fish (2-month-old, 0.80 − 0.95 g initial weight) were individually reared in 350-mL cylindrical plastic containers (8.3 cm diameter × 12.5 cm height). The fish were reared for 8 weeks in four concentrations of T. catappa leaf extract (0.125, 0.25, 0.5, and 1 g L−1) and a control group was reared in water without the extract. The fish received an excess of a commercial floating feed (39.4% crude protein) for fighting fish twice daily (08.00 and 17.00 h). A leaf extract concentration of 0.25 g L−1 showed no significant effect on growth (specific growth rate = 1.62% body weight day−1), feed utilization (feed conversion ratio = 1.21 g feed g gain−1), viscerosomatic index, muscle quality, whole-body composition, and hematological parameters of reared fish (P > 0.05), but did show effects on some of the other metrics that were monitored. Water quality significantly improved during the experiment (P < 0.05). Increased skin lightness (L*) was observed in all experimental groups, but only fish reared in 0.125 g L−1 leaf extract increased skin redness/greenness (a*), hue (h*), and chroma (C*) (P < 0.05). Relative to the control, increments in bubble nest formation frequency (1.70-fold on average) and bubble nest size (1.52-fold on average) were correlated with the concentration of T. catappa leaf extract (P < 0.05). Specific activities of protein- and lipid-digesting enzymes were not negatively affected but some modulations were observed in treatments containing leaf extract at 0.125, 0.25, and 1 g L−1. Fish reared in the 0.25 g L−1 treatment exhibited a sparing effect of carbohydrate utilization indicated by decreased amylase specific activity, increased lipase specific activity, and stable amylase/trypsin ratios. The results suggest that 0.25 g L−1 of T. catappa leaf extract is a suitable concentration of the additive during the growing phase of male Siamese fighting fish.
