One of the biggest challenges of recirculating systems (RAS) is keeping levels of toxic compounds, such as ammonia, low. This work sought to evaluate the use of eggshell as a biofilter substrate in the cultivation of juvenile Oreochromis niloticus. The experiment lasted 90 days, using four treatments: A) 100% expanded clay, A+CO) 50% expanded clay and 50% eggshell, PC) MBBR3 and NC) without substrate, with three replications each, storing 15 juveniles with an initial weight of 1.26±0.08g. Feeding occurred three times a day with commercial food offered until apparent satiety. The following parameters were monitored: pH, temperature (ºC), dissolved oxygen (mg L-1), electrical conductivity (µS cm-1) and total soluble solids (ppm); and zootechnical performance indices: survival rate (%), weight gain (g), specific growth rate (% day-1) and condition factor. Water samples were collected at the inlet and outlet of the biofilters on days 0, 30 and 60 of the experiment. Means were compared using the 5% Tukey test. All biofilters used maintained water quality parameters within those recommended for production, with emphasis on A+CO. The concentrations of ammonia at the inlet of the A+CO biofilter were lower than the others, providing lower levels of this substance in the system and high concentrations of nitrate. It was concluded that the biofilter composed of 50% expanded clay and 50% eggshell showed better efficiency in maintaining water quality compared to the others, revealing its potential as an alternative biological substrate.