Optimal crop production in aquaponics is influenced by water pH and potassium concentrations. The addition of potassium hydroxide (KOH) into the recirculating aquaculture system (RAS) may benefit aquaponics by increasing the water pH for better biofilter activity and supplementing K for better plant growth and quality. We investigated the growth, feed conversion, body composition and welfare indicators of juvenile African catfish (Clarias gariepinus) treated with four concentrations of K (K0 = 2, K200 = 218, K400 = 418, and K600 = 671 mg L⁻¹). While growth, feed conversion and final body composition were unaffected, the feeding time and individual resting significantly increased with increasing K⁺. The swimming activity and agonistic behavior were reduced significantly under increased concentrations of K⁺. Leftover feed and the highest number of skin lesions were observed under K600. We suggest that K⁺ concentrations between 200 and 400 mg L⁻¹ can improve the welfare status of juvenile African catfish. This enables the application of KOH in RAS to supply alkalinity to achieve optimum nitrification at minimum water exchange and improve the nutritional profile of the process water with benefits for the welfare status of African catfish and aquaponics plant production and quality.