The synergic, syntropic and metabiotic ability of a combination of lactic acid bacteria, phototrophic bacteria and yeast (also known as effective microorganisms – EM) to reduce residual water contamination is well recognised. Fish ponds’ organic matter (which would normally be eliminated) could be used as substrate for EM culture. Microorganisms can serve as both fish-food and reduce effluents in water bodies and concentrated food consumption. The effect of a commercial EM culture was evaluated on tilapia (Oreochromis sp) fry weight-gain. Fry (n=10) having an average 0.604 ± 0.059 g weight were placed in ten 25 l plastic containers in laboratory conditions for 2 weeks. Five containers were used as control (T1); the other five (T2) were inoculated daily with 2 ml of a commercial product consisting of Lactobacillus casei, Saccharomyces cerevisiae, Rhodopseudomona palustris, each having 106 colony forming units in watermolasses suspension. The feed provided was a commercial product having 40 % protein (feed ration was equal to 6 % of the fish’ initial weight). Container pH was kept stable at 6.7, temperature at 27°C and oxygen at 7 ppm. There were no water exchanges; there was permanent aeration. Treatment tank water was initially dark; however, when the assay finished all the containers had the same colour. Fish had lesions to their eyes, possibly due to the marked aggressiveness exhibited during the investigation. T1 had 0.7321 g ± 0.2126 weight-gain with 29.05 variation coefficient. T2 had 0.8034 g ± 0.095 weight-gain, 11.87 variation coefficient. No statistical difference was found (p<0.05).