The aim of this work was to evaluate the quality of RAS water, with daily breaks of 8 and 16 hours in the circulation of RAS water, during the rearing of juvenile tilapia (Oreochromis niloticus). The experiment was carried out over 50 days at IFES-Alegre, where 48 aquariums (35L) were used, with a filtration system for each battery of 12 aquariums, totaling four RAS, 420L useful. Each tank received 10 juveniles, with an average initial weight of 12.5g (3.6g/L). The RAS were subjected to two different water movement situations (treatments): T8H (8h stationary and 16h recirculating) and T16H (16h stationary and 8h recirculating). The following parameters were assessed: pH, dissolved oxygen, electrical conductivity, total dissolved solids and temperature, for 24 hours, every two hours, at 36, 43 and 50 days. Zootechnical parameters were also assessed: survival rate and weight gain. The water quality parameters behaved differently throughout the day and depending on the period during which the water was not recirculated.
The pH was close to 6, with less variation on days 36 and 43. On day 50, the pH values were lower than on the previous days, especially in T16H. Dissolved oxygen (DO) fluctuated according to the treatments, with T16H having a lower average concentration (3.45±1.4 mg/L) than T8H (4.70±1.7 mg/L). For T16H, the drop in DO was even more pronounced. There were no differences between the two treatments for electrical conductivity (EC), higher values were found throughout the day, which may be related to the amount of feces and leftover feed (during fish feeding). There were no differences in total dissolved solids (TDS) between the treatments. However, considerable differences were observed over the course of the experiment, with values rising from 394.3±4.5ppm on day 36 to 494.3±4.7 and 508.98±2.3pmm on days 43 and 50, respectively. Water temperature did not vary between treatments, remaining at 27.5±.03; 25.9±0.8 and 26.8±0.9ºC at 36, 43 and 50 days. Regarding the zootechnical performance of the juveniles, there were no statistically significant differences between the treatments, but there was a low survival rate of 66.8±22.6% for T8H and 58.6±25.1% for T16H.
We can be concluded that the changes observed in the RAS water parameters maintained for eight (T8H) and 16 hours (T16H) daily without water recirculation, mainly compromised pH and DO, which consequently affects the efficiency of the nitrification process, influencing above all the survival rate during the rearing of juvenile Nile tilapia.