Microplastic (MP) and nanoparticle (NP) contamination in fish feed threatens aquacultural productivity and food security. In this study we assessed the effects of dietary exposure to MP, NP, and their combination (MNP) on Cyprinus carpio growth and oral/intestinal bacterial composition. Contaminated feed significantly reduced weight gain and feed efficiency (p < 0.05), with MNP causing the most severe impacts on fish weight gain. Moreover, 16s rRNA sequencing revealed a significant shift in bacterial taxa, marked by pathogenic Aeromonas and Vibrio enrichment and commensal reduction, particularly in the intestines (p < 0.001). Beta diversity confirmed taxonomic shifts (PERMANOVA: R² = 0.248, p = 0.001), with intestinal bacteria more sensitive than oral bacteria. Aeromonas abundance correlated strongly with growth impairment (Mantel p < 0.05; R² > 0.5), indicating that it is a key growth-limiting pathogen and is elevated by contaminated feed. These findings highlight the dual risk of MNP-contaminated feed that disrupts the microbiome and decreases growth performance. Strategies to mitigate these effects are urgently needed to secure aquaculture feed.