One pound of phosphorus can support 500 pounds of algae or more. And the more phosphorus you have built up in your system, the more likely you're going to have nuisance types of algae, and a lot of algae on your hands.
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Phosphorous is usually the nutrient that supports algal growth the most, and your pond gets this nutrient from all kinds of sources (birds, lawn clippings, air, fertilizer). To remove and prevent algae, you need to remove the phosphorous. The worse the algae is, like green algae and blue-green algae, the more it likes phosphorous.
Algae before and after SeClear treatment.