Negative impacts of chemicals are documented in the environment, but a shortage exists in bioremediation studies for residues of such chemicals in aquatic systems. This study focused on the effects of Ethidium Bromide (EtBr) on fry of Oreochromis niloticus and the role of Spirulina platensis (SP) as a bioremediation agent. Five groups of fish were divided into (1) control, and four treatment groups: (2)10 µg L ^− 1 EtBr, (3)10 µg L ^− 1 EtBr with 200 mg L ^− 1 SP, (4)100 µg L ^− 1 EtBr, and (5)100 µg L ^− 1 EtBr with 200 mg L ^− 1 SP for 15 days. Antioxidant biomarkers–total antioxidant capacity; TAC, Superoxide dismutase; SOD, and catalase; CAT–were assessed. Exposure to EtBr led to a decrease in TAC after exposure to 100 µg L ^− 1 EtBr and 100 µg L ^− 1 EtBr +200 mg L ^− 1 SP. No significant change in SOD and CAT activity was observed compared with control fish, except for fish treated with 100 µg L ^− 1 EtBr. This treatment induces an increase in activity of SOD and CAT. DNA fragmentation increased significantly after exposure to EtBr alone and this increase was counteracted by combined exposure to SP. Exposure to EtBr induced alteration in antioxidant capacity and DNA damage. SP protects against these negative impacts.