Swimming pool water turbidity is usually overcome by 2 ways namely draining and administration of chlorine. Draining or administration of chlorine is not efficient because it needs a specified schedule and cannot be done outside the schedule, specifically for draining, when there are many visitors. The water turbidity needs to be monitored and detected to maintain water quality in the swimming pool. This monitoring tool is designed with a light sensor and its transmitter processed by using an AT89C51 type microcontroller based on Fuzzy Decision Tree Algorithm and an LCD. What is measured in this device is the strength of the light intensity when the lamp is blocked by water. This device is useful for knowing the right time to replace the pool water. The device was proven able to detect turbidity of the pool water. It can identify three pond water conditions namely; clear, medium and cloudy indicating the right the time to replace the pool water.