FC below 3 is too low for a 35 cya. Given that, there is a good chance it is algae. With CYA=35 your FC should be maintained between 3 and 5ppm. To shock, you need to bring it up to 15 or higher. You should keep it at 15+ until you can go from sundown to sunup losing less than 1ppm FC.


Here's the chart that shows the chlorine/CYA relationship. http://www.poolsolutions.com/gd/best...e-chart.html#a