Managing density is critical in farming sandfish (Holothuria scabra), significantly affecting their growth and survival. Larval sandfish are initially reared in high-density fiber tanks, but as they develop into juveniles, they require larger and more spacious tanks to support growth. This study observes and analyzes the growth performance of H. scabra juveniles reared in concrete tanks with a diet of Ulva lactuca. Approximately 200 hatchery-produced sandfish juveniles, aged 40 days with a mean length of 1.03 ± 0.43 cm, were reared in Netlon cages (16 m²) and placed in a large concrete tank with sandy sediment. They were fed weekly with 500 g of macroalgae U. lactuca. During the first 30 days, the juveniles showed slight growth to 3.20 ± 0.93 cm, further increasing to 4.82 ± 1.08 cm and 4.89 ± 1.13 cm at 60 and 90 days of rearing, respectively. The specific growth rate (SGR) calculated on day 90 was 1.75 ± 0.21% day⁻¹, and the survival rate recorded was 41%. In conclusion, the use of Netlon cages demonstrated positive results in sandfish growth and their suitability for sandy sediment conditions at the bottom of the tank, replicating their natural habitat. Additionally, the findings indicate that feeding sandfish with seaweed powder from the U. lactuca species contributes to improved growth.