I have a 20,000 gallon inground plaster pool. Water is clear with nice blue tinge. FC is at 0 no matter how much granular shock I put in. Now mustard yellow algae started growing on the walls... I can’t get the chlorine level up... please help!
Can you post other test results for CYA, CH, pH, etc? How long has it been testing at 0 FC? Do you manually add chlorine (which type - pucks or liquid) other than the granular shock you have added. Do you have a SWG? How many hours a day is your pool pump circulating? This and any other info would help determine next course of action.
Welcome to TFP! No chlorine means the water is potentially unsanitary and probably growing algae which makes sense based on what you are describing. To kill algae and remedy the low FC situation, follow the SLAM Process by using one of the recommended test kits like a TF-100 (link below) or Taylor K-2006C - see Test Kits Compared. It all starts with the proper test kit. See our ABCs of Pool Water Chemistry for more info, and don't forget to update your signature as well. Have a good day.