Macroalgae (seaweed) can be cultured effectively for the production of useful algal biomass and removal of nutrients from fishpond effluents. A land-based, tide/gravity-driven flow-through, fish-macroalgae integrated system was studied at Makoba Bay, Zanzibar, Tanzania, during May–October, 2000. Rectangular cages made of 1-inch mesh netting were constructed in channels that received the outflows of the fishponds. Four species of macroalgae were planted in the cages and compared for their usefulness as biofilters. Gracilaria crassa and Ulva reticulata grew at average rates of 1.5 and 1.2 %, respectively. Both species removed nitrogen as seaweed protein at rates of up to 0.4 g N/m2/d. The algal biomass produced was of good quality with protein dry weight contents of 13% for G. crassa and 26 % for U. reticulata. The biofilters also raised the pH values of the fishpond effluents and oxygenated the water. In contrast to Ulva and Gracilaria, species of Eucheuma and Chaetomorpha performed poorly in the fishpond effluents.