Background:
Aquaculture intensification generates nitrogenous and phosphorus-rich effluents that threaten aquatic ecosystems. Fermented plant-based feeds are increasingly used to enhance nutrient digestibility and protein availability, yet their impact on effluent water quality remains poorly understood. Understanding how substrates such as banana, jackfruit seeds, and sweet potato influence ammonia, nitrite, BOD, and phosphates is critical for developing sustainable feeding strategies and minimizing environmental pollution in intensive African catfish (Clarias gariepinus) culture.

Objectives:
This study aimed to evaluate the effects of solid-state fermented ripe banana, jackfruit seeds, and sweet potato tuber feeds on effluent water quality in African catfish (Clarias gariepinus), focusing on ammonia, nitrites, phosphates, BOD, copper, EC, and microbial composition to identify environmentally safer feed options.

Methods:
African catfish (Clarias gariepinus) fingerlings were stocked in 50 L glass aquaria and fed either fermented ripe banana, jackfruit seeds, sweet potato tubers, or commercial feed as control. Each treatment was triplicated in a completely randomized design. Effluent water was sampled weekly for four weeks to measure total ammonia nitrogen (TAN), nitrites, phosphates, biochemical oxygen demand (BOD₅), copper concentration, electrical conductivity (EC), pH, and microbial composition. TAN and nitrites were determined using colorimetric HS aqua test kits, phosphates and copper via Palin 7100 photometer, BOD₅ with a magnetic stir BOD system, and microbial counts on nutrient agar. Statistical differences were assessed using Kruskal-Wallis and Dunn's post hoc tests (p < 0.05).

Results:
Effluent water from tanks fed fermented banana and sweet potato exhibited lower total ammonia nitrogen (0.2–0.3 mg L⁻¹) and nitrites (0.01–0.12 mg L⁻¹) compared to jackfruit seeds (TAN 1.3 p < 0.05 mg L⁻¹, nitrites 0.75 p < 0.05 mg L⁻¹) and commercial feed (TAN 2.7 mg L⁻¹, nitrites 0 mg L⁻¹). Phosphate concentrations and biochemical oxygen demand (BOD₅) exceeded regulatory limits in all treatments except partial reduction in sweet potato tanks. Copper concentrations and electrical conductivity remained below permissible limits across all feeds. Microbial analysis revealed dominance of Bacillus and Lactobacillus species, with highest Bacillus counts in jackfruit seed tanks and Lactobacillus in banana tanks. Kruskal-Wallis tests confirmed significant differences (p < 0.05) among treatments for TAN, nitrites, phosphates, BOD₅, copper, EC, and microbial counts.

Conclusion:
The study demonstrated the potential of specific fermented plant-based feed ingredients to mitigate nitrogen pollution in aquaculture systems. In particular, the inclusion of fermented ripe banana and sweet potato tubers in fish diets was shown to reduce ammonia and nitrite concentrations in culture water and effluent. However, high BOD and phosphate persisted, revealing a knowledge gap on nutrient release and effluent dynamics, guiding future sustainable feed research.