This paper presents a design of an electronic system for maintaining optimum water quality in fish culture in the tropics. The aim of the work was to design an automation system that will maintain optimum water quality at low cost in tropical fish culture. Top-down design approach was adopted for designing the system. The subsystems or subunits of the optimum water quality maintenance system are power supply, sensing, control, water exchange, and communication and display units. Arduino Mega 2560 was utilized in this design for controlling the activities of the entire system. The system takes preventive measures against water quality problems in fish culture by weekly exchange of 10-20% of the water and replenishment of pond whenever water level decreases. The automation system controls pond water quality by exchanging 20% of the water with appropriate water whenever turbidity, temperature, dissolved oxygen or pH level reaches optimum limit. The system also sends an SMS message to the pond manager/farmer mobile phone to inform him of water quality condition and instruct him to erect sun screen in pond during summer and wind shelter during winter. The electronic model provides an efficient, low-cost automation method for maintaining optimum water quality and improving environmental conditions in tropical fish culture. The authors recommended that the system be developed and used in tropical fish culture in order to maintain optimum pond water quality and achieve optimal fish production at low cost.