I think you need a multi ...

I think you need a multi barrier approach for this. First you need to reduce nutrient levels in your water and especially phosphorous. A proper coagulant dose ahead of your sand filter can lower phosphorous concentration. Another way is to biologically activate your media filters to consume the nutrients which might help with the problem. Finally you need to dose chlorine ahead of your product tank and prevent light from entering it to limit algae growth. Also make sure to properly clean and disinfect the tank before reusing it as adding chlorine to the tank with too much algae into it can release intercellular algal compounds which can be harmful.