The study aimed to determine the effect of adding potassium sulphate (K₂SO₄) at a concentration of 500 mg/dm³ on the quality of the aquatic environment, growth of Clarias gariepinus and yield of lettuce in an aquaponics system. The experiment was conducted in two parallel lines: control (without K₂SO₄) and experimental (with the addition of K₂SO₄). The initial total weight of 19 fish in each line was 1330 g. In 57 days, the weight in the control line increased to 1995 g, and in the experimental – to 2888 g. The feed conversion was 2.5 in the control and 1.1 in the experiment. The specific growth rate (SGR) was 0.82%/day in the control and 1.74%/day in the experimental group. The potential yield of lettuce (4000 g/m²) was realised by 51.1% in the control and 87.5% in the experiment. In the control line, the main limiting factor was potassium deficiency (21.6-28.3 mg/l), and in the experimental line, the pH increased to 8.3 due to insufficient nitrogen uptake at 19.5 °C (below the optimum for nitrification). In both lines, a significant increase in the content of mineral compounds (NO₃-, NH₄⁺, SO₄²-, PO₄³-) was recorded, but the water parameters remained within the range acceptable for aquaponics technologies. The results obtained indicate a positive effect of adding K₂SO₄ at a given concentration, which ensures an increase in fish and plant productivity, although it requires the incorporation of the ratio of green crops to fish biomass and water temperature. The addition of K₂SO₄ contributed to the increase in the biological efficiency of the aquaponic system while maintaining acceptable water quality
lettuce, biomass growth, feed conversion, water quality, nitrate nitrogen, hydroponics module performance