Do phosphates matter in a pool?

Compounds of phosphorus, a nutrient that plants and algae need to grow. You cannot see or smell them. A phosphate test reads them in parts per billion (ppb), not parts per million like the other tests.

Mostly from the yard. Lawn fertilizer, grass clippings, leaves, pollen and dirt all carry phosphorus, and every storm washes some of it in. UF/IFAS points out that rain runoff carries grass clippings and fertilizer with it, and around a pool that runoff ends up in the water. Some fill water and some pool products add a little too.

No, low chlorine does. Phosphates are food for algae, but algae cannot take hold in water that keeps enough free chlorine to kill it. That is why a pool with steady chlorine stays clear even with phosphates in it, and a pool that lets the chlorine fall to zero turns green with or without them. See why is my pool green.

When the basics are already right and algae keeps coming back anyway, or when the level tests very high, say after a lot of yard runoff. Fix chlorine, pH and stabilizer first. A remover will not save a pool where the chlorine keeps running out. Keeping the chlorine from running out is a big part of what our weekly pool service does, and every chemical is included.

Because it works by binding phosphates into fine particles that the filter then has to catch. Some cloudiness for a day or two is normal. Keep the pump running and clean the filter afterward, since what it traps clogs the filter quickly. More in why is my pool cloudy.

Keep the phosphorus from getting there:

• Keep fertilizer and clippings off the deck and blow them away from the pool, not into it.
• Aim sprinklers away from the water.
• Net leaves before they sink and break down.
• Keep the filter clean, and keep the screen enclosure in good repair so less blows in.

Yes. Phosphates are not a swimmer health issue. What matters for swimming is that chlorine and pH are right; the CDC minimum is 1 ppm free chlorine, or 2 ppm with stabilizer.