Phosphates

Removing them

Lanthanum-based phosphate removers bind the phosphates into a precipitate that the filter catches. Expect the water to cloud for a day or two, and expect to clean the filter afterward. Dose to the product label, since the strength varies a lot between brands.

If the source is still there, the reading climbs straight back. Fixing the runoff or the debris is the durable answer, and it's worth saying to the homeowner rather than selling remover every quarter.

Phosphates are algae food. They don't hurt the water, the equipment or the swimmers. They just make it easier for algae to take hold when chlorine slips.

Chlorine first

A pool with correct chlorine for its stabilizer level doesn't get algae, whatever the phosphate reading says. If a pool is green, chlorine is the reason. Phosphates explain why it went green fast.

So check the FC–CYA ratio before you sell a phosphate remover. Selling one to a pool that's actually short on chlorine treats the wrong problem and the algae comes back.

When it is worth testing

• Algae keeps returning on a pool whose chlorine you know is right
• Chlorine demand is high with no other explanation
• The pool is surrounded by planting that gets fertilized
• After a lot of runoff into the pool

Where they come from

• Lawn and garden fertilizer washing in
• Decaying leaves and other organic debris
• Some fill water, especially well water
• A few pool chemicals, including certain stain and scale products