As growth-promoting feed additives, compound acidifiers (CAs) have been widely reported in many farmed fish species, whereas there is little information on the application of CAs in the eel diet. The present trial was conducted to evaluate the supplementation effects of CAs on growth performance, serum biochemical parameters, and body composition of the juvenile American eel (Anguilla rostrata). The CAs supplementation levels in the diet of American eel were 0, 2.0, 3.0, 4.0, and 5.0 g/kg, respectively. The trial lasted for 12 weeks. The most beneficial effects of CAs on promoting growth performance, decreasing levels of serum lipid, modifying parameters of the liver function and immune function in the serum, and increasing the utilization of calcium and phosphorus in the diet were observed in the fish fed with 4.0 g/kg CAs. Dietary CAs supplementation had no significant effects on the body composition of whole fish, whereas 5.0 g/kg of dietary CAs supplementation exerted negative effects on growth performance, serum biochemical parameters, and utilization of calcium and phosphorus. The recommended level of CAs in the diet was 3.5–3.7 g/kg to achieve the optimal weight gain and feed conversion ratio for the juvenile American eel.