Phosphates

Phosphates come in with fill water, fertilizer, leaves, and rain. Algae uses them as food. High levels do not cause algae alone, but they make it easier for algae to take hold when chlorine dips.

If algae keeps coming back even with steady chlorine, a phosphate test is a fair next step. A remover product can lower the level, but it works best alongside good chlorine and filtration. Keep lawn fertilizer and grass clippings out of the pool. Do not expect a remover to fix a pool with poor circulation or a worn filter.