Abstract This investigation examined the effects of light intensity, stocking density, feeding frequency and salinity on the growth of sub-adult seahorses, Hippocampus erectus and the development of the brood pouch in male seahorses. During the 8-week study, seahorses cultured at a light intensity of 1000 lx had higher incremental wet weight and standard length than those at 100 lx and 2500 lx. Seahorse growth rate decreased with increasing stocking density (0.25, 0.5, 1 and 1.5 inds/L, respectively) during the 10-week study. Male seahorses grew much faster than females, with incremental wet weights of male and female seahorses of 6.64 ± 1.601 g and 5.87 ± 1.173 g, respectively, in the 0.25 inds/L treatment. The brood pouches of the males emerged earlier, and the ratio between the pouch length and standard length was higher at the higher stocking densities (1 and 1.5 inds/L). The final body weight, weight gain (WG) and specific growth rate (SGR) of the male and female seahorses that were fed 4 times per day were the highest, while those fed 3 times per day had the highest standard length after 10 weeks of culture. The ratio of the length of male brood pouch to standard length was the highest in the low feeding frequency treatment (fed once a day). The incremental wet weight and standard length of sub-adult seahorses differed significantly among the different salinities (35, 33, 31, 29 and 27‰) after 8 weeks of culture ( F 4,60 = 18.950, F 4,60 = 9.225, P < 0.05) with greatest growth at 31–33‰. Results demonstrated that appropriate light intensity, stocking density, feeding frequency and salinity can improve the growth rate of the sub-adult seahorses and affect the development of the brood pouch in male seahorses.