In most land-based mariculture, seaweeds have been cultured using expensive ponds and electrical pumps to aerate the seaweeds, methods that can be expensive and not applicable to areas where electricity is lacking. In this study, a low-tech, low-cost, gravity-generated water flow regime was used in earthen ponds in Zanzibar, Tanzania. The seaweed Ulva reticulata was cultured suspended in 2 m−2 fishnet cages (units) in channels located at the outflow of fishponds as treatment and at the channel leading water to the fishponds as control, in a fish-seaweed integrated system. The seaweed grew at an average of 4.0% per day at the treatment (fishpond outflow channel) with a biomass yield averaging 46 g m−2 day−1, compared with 2.5% and 27 gm−2 day−1 at the control (fishpond inflow channel). Likewise, the seaweed removed a significant amount of areal total ammonia nitrogen (TAN) averaging 6.5 g Nm−2 day−1 with 65% removal efficiency and 0.3 g Pm−2 day−1 of soluble reactive phosphate (SRP) with 33% efficiency, from a TAN load of 10.6 g m−2 day−1 and phosphate load of 0.7 g m−2 day−1. In the control, only 1.9 g Nm−2 day−1 (44% efficiency) TAN and 0.3 g Pm−2 day−1 (33% efficiency) SRP were removed from TAN and phosphate loads of 4.8 and 0.8 g m−2 day−1, respectively. Through its photosynthetic activity, the seaweed significantly (pb0.001) raised the oxygen concentration and pH of the fishpond outflow water (from 5.6 to 13.4 mg l−1 and 7.8 to 8.4, respectively). It is therefore concluded that seaweeds can be cultured in fishpond outflow channels, in a low-tech, low energy, gravitygenerated water flow regime and function as effective biofilters of fishpond effluent water. © 2005 Elsevier B.V. All rights reserved.