This study investigates the impact of fish stocking density on water quality, red tilapia (Oreochromis sp.) growth, and mustard green (Brassica integrifolia) growth in microalgae-aquaponics systems. Three densities were tested: D1 (90 fish/m³), D2 (70 fish/m³), and D3 (50 fish/m³). Water quality monitoring showed that TAN, NO₂⁻, and NO₃⁻ levels increased with fish density, while phosphate levels remained relatively stable. D1 had the highest total fish weight gain but a lower weight gain rate (91.9 %) compared to D2 (105.6 %) and D3 (134.1 %). D3 had the highest average weight gain per fish (67.78 g) and the best survival rate (100 %), whereas D1 had the lowest survival rate (94 %). Mustard greens grew best in D1. Microalgae density increased during experimental time, with D3 showing the highest biomass in both the algal and fish tanks. The results suggest that higher fish densities promoted plant growth, while lower densities benefited fish and microalgae, thus offering insights for optimizing recirculating microalgae-aquaponics systems.