This paper proposes an evaluating, monitoring, and warning system for water quality for taking care of fish that includes an Arduino Mega2560 board and a sensor system with wireless communication technologies based on IoT technology. Not only the temperature but also the pH and dissolved oxygen (DO) content of water are acquired through the Arduino Mega2560 board. These measurement data are transferred to the server through ESP8266 and SIM800A modules integrated on the board. The stored temperature, pH, and DO values are displayed on a ThingSpeak server. Furthermore, the fault detection of water quality under real working conditions is handled and displayed on the platform or a cellphone-based web service and short messaging service (SMS). The accuracy and reliability of the system for the functions of monitoring, evaluation, and fault detection were demonstrated. Compared with established systems, the proposed system has the advantages of (1) reduced cable use for the monitoring system, (2) a warning system via a cellphone-based SMS cellphone or web service, (3) cost-effectiveness, and (4) portability.