What do you mean I have a high phosphate level in my pool? Why did that cause algae?

Phosphate is a natural component of most swimmer wastes. It is also present in rain water, tannic acid in leaves and pollens.  Phosphate is persistent and does not break down naturally. Landscape fertilizers, which may blow into the pool, also have high phosphate content. For these reasons, pools can quickly build up high phosphate levels. This creates an abundant and tasty food source for all strains of algae, and can make controlling their growth difficult. Remove the food, and you have a strong weapon against algae.