However, the efficiency of the nitrification process presents a high degree of fluctuation, as the bacteria are influenced by environmental disturbance and changes in operating conditions (Malone and Pfeiffer, 2006; Graham et al., 2007). This situation arises because fish farmers must constantly change their fish cul- ture strategies according to the biomass or the number of fish present in the RAS. Normally, the RAS system is designed according to the maximum possible level of biomass the system can pro- duce, and this design is made prior to the installation. The problem appears when the fish culture produces lower biomass than was originally planned, which is a strategy used by fish farmers when prices are low or because other options or species are being intro- duced. This change in the level of biomass present in the RAS means the quantity and quality of water also changes, and although the system can be adapted to some degree of change, it is not possible to affect changes as fast as is necessary. This perturbance means addi- tional stress on the bacteria because the substrate is not constant, and this therefore affects the efficiency of the biofilter. A dynamic system like membrane process would clearly be better in these cir- cumstances, in which it was possible to adapt the system rapidly in accordance with the level of biomass in culture.
Stress also affects the species in culture, since total ammonia nitrogen (TAN) compounds cannot be maintained at an adequate level (Svobodova et al., 2005).
High levels of nitrite have toxic effects (Malone and Pfeiffer, 2006; Cheng et al., 2004), due to increased alteration or chronic diseases (Emparanza, 2009). Ammonia concentrations between 30 and 300