Actually, 500 ppb is not bad; the water here has over 2,000 ppb!

If you'd like, go ahead and pick one of the removers (Orenda PR-10000 or the Kem-Tek product), purchase it, and take the phosphates below 125 ppb. BUT, keep your chlorine up. Low phosphates weaken, but do not kill algae.

-- I assume you aren't using any products that would ADD phosphates, like stain removers, United Chemicals products, etc.? --

How's your pool looking, or more specifically, how is the mustard algae looking?