Effective culturing and restocking of any commercial holothurian are dependent on understanding the biology and ecology of the species throughout its life history. Inherent differences between species require distinct strategies in larval and juvenile rearing. This study evaluated the effect of macroalgal and seagrass detritus as supplemental food on growth, feeding and sheltering behaviour, and biochemical composition of cultured Stichopus cf. horrens juveniles (2.6–3.8 g) under nursery rearing conditions. The addition of detritus to sediments did not significantly affect juvenile growth and body composition when compared to a biofilm-only control. Average specific growth rates were positive throughout the experiment (1.58%–7.88% per day), however size variability, feeding rate, and sheltering behaviour differed significantly. Juveniles in treatments with macroalgal detritus had the highest coefficient of variation in weight (40%–51%) and a propensity to avoid sheltering near the sediments. This is presumably due to anoxic sediment conditions that developed in the macroalgal treatment over time. Juveniles in detritus treatments exhibited lower faecal production rates at the beginning of the experiment and after 75 days without significant differences in growth, suggesting the availability of high-quality food in these treatments. This study provides key insights on husbandry conditions highly relevant to further develop S. cf. horrens aquaculture.