•The growth was decreased with increasing of both forms of Cu doses.•Copper was increased in all tissues with increasing of both forms of Cu doses.•The remaining tissues and the liver contributed most of the total Cu in the body.•Either Cu-NPs or CuSO4 caused tissues oxidative stress and cell apoptosis.•Dissolved Cu was more toxic than Cu-NPs. Copper nanoparticles (Cu-NPs) were widely used in various industrial and commercial applications. In this study the effects of Cu-NPs and soluble Cu were investigated on juvenile Epinephelus coioides. The fish were exposed in triplicate to control, 20 or 100μgCuL−1 as either copper sulphate (CuSO4) or Cu-NPs in a semi-static aqueous culture for 25 days. The growth parameters were significantly lower at 100μgCuL−1 as CuSO4 or Cu-NPs treatment compared to control. Time-dependent Cu accumulation in all tissues increased with increasing the Cu dose. The percentage of total Cu found in remaining tissues (head, bones, fins, etc.) decreased more in the CuSO4 than Cu-NPs treatment after 25 days, but increased in all other tissues (especially in liver). Compared with the control, either Cu-NPs or CuSO4 induced higher malonaldehyde concentration in tissues by overwhelming total superoxide dismutase activity, total glutathione concentration and Na+/K+-ATPase activity, but the opposite results were recorded for the brain. With increasing the CuSO4 or Cu-NPs dose, apoptosis was exacerbated in the liver and gills, more so by CuSO4 than Cu-NPs. Overall, these findings showed that Cu-NPs had the toxic effects similar to dissolved Cu; hence, Cu-NPs need to be included in the assessment of toxicological impacts in the aquatic environment.