An anaerobic denitrification module with improved stability for closed aquaculture systems is described. The module was fitted to a recirculating fish mariculture system. The 18 m3 system consisted of a fish basin, a nitrification trickling filter and the module reported here, which was composed of a stirred slurry basin and two subsequent sedimentation tanks. Commercial fish feed (pellets) introduction into the slurry basin simulated nutrient input equivalent to 450 kg of fish biomass. About 50 kg of the sea bream, Sparus aurata, were introduced to bioassay water quality. Conditions remained stable as long as the mixing remained regular (16 wk). Feeding and sampling continued for an additional 6 wk of irregular mixing, during which time water quality gradually deteriorated causing 19% mortality of the stocked fish population. The system removed a majority of the total dissolved nitrogen (N) by denitrification and total phosphorus (P) by immobilization in the sludge. The rate of denitrification approached 9 mmol N/L/d. Hydrogen sulfide concentration in the fish basin remained low. The results constitute a proof-of-concept for the utility and resilience of slurry basins in the control of anaerobic processes and the maintenance of good water quality in recirculating fish culture.