Fish production in terrestrial recirculating aquaculture systems (RAS) has steadily increased over the past decades. Therefore, several studies, both in freshwater and seawater, have shown that increased water reuse in RAS leads to increased levels of heavy metals in the bodies of fish. The objective of the present study was to evaluate the impact on the physiology, osmoregulatory mechanisms, and antioxidant response in fingerlings of rainbow trout (Oncorhynchus mykiss) exposed to varying doses of AlCl3 within a recirculation system. Changes in gills, N+-K+-ATPase activity and H+-ATPase activity, plasma osmolality, lactate, glucose, chloride levels, and gills and liver oxidative stress responses were used to monitor physiology responses. As a result, AlCl3 exposure modified responses in rainbow trout. Plasma parameters increased during the 6-day exposure, while lactate levels decreased in both doses. The enzymatic activities of antioxidant enzymes and the NKA pump increased when fish were exposed to high and low doses of aluminum chloride. Gene expression of glutathione reductase and peroxidase transcripts increased and decreased respectively in the liver, contrary to that observed in gill tissue. AlCl3 was also observed in liver and gill tissues. This study shows that exposure to AlCl3 affects ion regulation and oxidative stress in rainbow trout fry.