<![CDATA[Koi fish (Cyprinus carpio) are ornamental fish with high economic value and visual appeal. Water quality is one of the key factors affecting koi health and condition and can be monitored through parameters such as temperature, pH, and Total Dissolved Solids (TDS). This study aims to implement an Internet of Things (IoT)-based water quality monitoring system for koi ponds capable of acquiring, processing, and presenting data in real time via a smartphone. The research employed a Research and Development (R&D) approach consisting of literature review, requirements analysis, system design, implementation, calibration, testing, and evaluation. The system uses a PH4502C pH sensor, a DS18B20 temperature sensor, and a TDS sensor connected to an Arduino Uno for data processing, with an ESP8266 serving as the Wi-Fi communication module. Measurement data are transmitted to the Blynk platform and displayed through a monitoring interface. Testing was conducted through five measurements within a one-minute period and compared with measurements obtained from reference instruments. The results showed average system readings of 25.46 °C, 6.91, and 191.40 ppm, with standard deviations of 0.055 °C, 0.010, and 1.67 ppm for temperature, pH, and TDS, respectively. Compared with the reference instruments, the system yielded mean differences of 0.02 °C for temperature, 0.01 for pH, and 2.00 ppm for TDS. The relatively small standard deviations and low mean differences indicate consistent system readings under the tested conditions. The system also successfully integrated data acquisition, processing, communication through the ESP8266, and visualization through Blynk, enabling real-time monitoring of water quality information. These results demonstrate that the system can serve as a prototype for multi-parameter water quality monitoring, while further evaluation of absolute accuracy, data transmission reliability, packet loss, and latency requires additional testing using reference instruments and dedicated communication logging.]]>
