Phosphates
Also called: phosphate, phosphate remover.
Phosphates are algae food that gets into every outdoor pool through fill water, leaves, pollen and lawn runoff. Phosphates do not cause algae when free chlorine is held at the right level for the pool's cyanuric acid (CYA), so a normally run pool does not need phosphate remover.
Why it matters
Food doesn't matter if the algae can't live. Trouble Free Pool documents crystal-clear, algae-free pools running well over 4,000 ppb of phosphates. A high reading on the store's meter is the default state of an outdoor pool. Phosphate remover uses lanthanum to turn phosphate into an insoluble solid, a fine cloud your filter then has to catch, and an overdose leaves a persistent milky haze. Many metal sequestrants are phosphonate-based and break down into phosphates. The narrow real cases are a pool you can't keep chlorinated and a salt pool at extreme phosphate levels.
In numbers
Algae showing up alongside a high phosphate reading means chlorine is too low for your CYA. SLAM it, don't de-phosphate it.