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Experiments were conducted to test the usefulness of biofilms—a microbial consortium associated with extracellular polymeric substances attached to submersed surfaces—in reducing the levels of ammonium and phosphate of rearing system water, and as a food source for the shrimp Farfantepenaeus paulensis. A mature biofilm, which is able to keep ammonium and phosphate at low levels, occurred 10–15 days after tank cleanup, and was characterized by chlorophyll-a concentration around 5 μg/cm². It was mainly composed of pennate diatoms (Amphora, Campylopyxis, Navicula, Sinedra, Hantschia and Cylindrotheca; ca. 9×10⁴ cells/mg of biofilm) and filamentous cyanobacteria (Oscillatoria and Spirulina; ca. 2×10⁵ cells/mg), though bacteria (max. 1.48×10⁷/mg), flagellates (max. 1.08×10³/mg) and ciliates (max. 3.51×10²/mg) were also present. Pennate diatoms and filamentous cyanobacteria were …