The condition of the sea and coastal areas is getting worse every day. Poor sea conditions will damage the marine ecosystem because the quality of sea water is a very important factor for the survival of coral reefs and other marine biota. To reduce the impacts that occur due to poor water quality, a monitoring and control system is needed so that water quality can be controlled properly. Therefore, the author makes a coral reef ecosystem monitoring system using the Fuzzy Logic method, the system will monitor the pH and turbidity of water, then use fuzzy as a decision support method to determine the quality of sea water.