This study examined how varying ammonium-nitrogen (NH4–N) concentrations combined with a gradient of redox potentials (oxic to anoxic) affect growth, nutrient accumulation and biochemical stress markers (e.g., MDA, H2O2, proline, antioxidant enzymes) in the aquatic macrophyte Elodea nuttallii. Results showed that reduced redox conditions and elevated NH4–N levels negatively affected growth and increased oxidative stress indicators and certain element accumulations.