Koi (Cyprinus carpio L.) are a highly popular aquaculture commodity in Indonesia, particularly in Blitar District, where they are primarily cultivated using a semi-intensive system supported by natural plankton. This study aimed to identify the plankton community and to quantify its abundance as a bioindicator of water quality in Koi ponds. The research was conducted in March 2024, with a total of 20 water samples collected from 10 sampling stations—comprising both rearing and grow-out ponds—during the morning and afternoon. Several water quality parameters were measured both in situ and ex situ, including dissolved oxygen (DO), pH, and nitrite levels. The data were analyzed using canonical correspondence analysis (CCA). Results showed that the plankton community was dominated by the Chlorophyceae class, accounting for 48% of the total. Phytoplankton abundance reached 35,724 cells/L, significantly higher than the zooplankton abundance, which was 368 individuals/L. A total of 31 phytoplankton species and 9 zooplankton species were identified. The biological index indicated a medium to high diversity, with high evenness and low dominance. CCA analysis revealed a correlation between plankton abundance and various water quality parameters. Based on these findings, enhancing the presence of Chlorella and Nitzschia could support the natural diet of Koi and potentially enhance their body coloration.