This research aimed to improve the quality and increase the yield of red tilapia in the biofloc system. Experiment 1 was to study the digestibility efficiency of In vitro cellulose digestibility and protease inhibitor activity including the percentage of protease activity. Inhibition was conducted to investigate the appropriate conditioning of napier grass to produce red tilapia feed. The cellulase crude extracted from intestine of red tilapia (22 – 25 g/fish) were determined the in vitro cellulase digestibility, the protease inhibitor activity and percentage of protease inhibition from fresh, fermentation (10 days with anaerobic), sun for 12 hours and boiled (10 minutes) napier grass. The results showed that cellulase digestibility were highest in sunny napier grass (P<0.05). In addition, percentage of protease inhibition and protease inhibition activity was lowest in sunny and boiled napier grass (P<0.05). Experiment 2 was the study of the microenvironment and water quality factors in red tilapia cultured in the biofloc system only, in biofloc system with starch and in biofloc system with Napier grass. It was found that there were 10 species of phytoplankton, 4 Divition, and 6 species of zooplankton, 3 Phylum. The important water quality factors obtained in all trials was in the safe criteria. For growth performance, the average weight, the daily growth rate (ADG) of red tilapia rearing in biofloc and napier grass was likely the best. Although the value is not statistically different from those rearing in biofloc with tapioca starch. However, both culturing trials were significantly better than biofloc-only experiment (P <0.05).