Background: Reduced graphene oxide (rGO) is a carbon nanomaterial with specific properties, which allow its use in several areas. Some studies have characterized the effects of rGO on aquatic organisms, but the ability to recover after exposure remains largely unknown. Aim: In the present study, the toxicity of reduced graphene oxide (rGO) was studied in zebrafish (Danio rerio) using biomarkers of oxidative and metabolic stress, immunological dysfunction, and cellular damage. The gill tissues were morphologically assessed and also the best dose of carvacrol (CVC) for neutralizing adverse effects was analyzed. Methodology: Fish were fed for fourteen days, and eight study groups were investigated: control, rGO exposure alone (10 mg L−1) and combined with three different carvacrol doses (10 mg L−1 of rGO + carvacrol 100, 150 or 200 mg kg−1 of food). Result:D. rerio exposed to rGO alone or in combination with a lower carvacrol dose demonstrated overall inferior results compared to control groups and those supplemented with rGO and 150 or 200 mg kg−1 carvacrol. rGO impairs cells by increasing LDH activity and cortisol levels and inducing oxidative stress by reducing SOD and CAT activity and suppressing the immune system through lysozyme activity. Lesions in gill tissues included cells with peripheral nuclei. rGO exposure increased apoptotic and necrotic gill cells. The structural study of the gill tissues revealed multiple lesions, including a dilated marginal channel, lamellar fusion, clubbed tips, swelling mucocytes, epithelial lifting, aneurysms and necrosis. Conclusion: As a result of our findings, we believe that a sub-lethal dosage of rGO could be hazardous to fish species, posing a threat to the aquatic food chain. Although rGO is harmful to the groups exposed to it, CVC supplementation at 150 or 200 mg kg−1 can protect against its toxic effects.