Why to use
Phosphate Removers should be looked at as an insurance policy NOT as a required part of pool care. If a user struggles to maintain the FC/CYA ratio then using a phosphate remover may help the user avoid an algae outbreak. Algae will occur if chlorine isn't added, but with a low phosphate level it'll take longer to happen.
Testing
The goal is always zero phosphates or as low as possible. So it doesn't really matter how high they are to start with or where the test maxes out at. For the Taylor test ($11 by the way), there's a high limit version that goes up to 2000ppb but it's not necessary, just use water dilution if you max out to get a feel for where you are. Most commercial grades of PO4 removers can handle 10,000ppb per 10,000 gallons of pool water per quart of of product administered.
Using Phosphate Removers
Lanthanum chloride reacts with the free, dissolved phosphate compounds to form a salt (precipitate) of Lanthanum and phosphate (lanthanum phosphate) which is not soluble in water. The chlorine ion from the Lanthanum chloride likely ends up bound to free sodium ions to form sodium chloride, table salt, which would dissolve in water. Lanthanum sulfate is also in these phosphate removers and reacts in a similar way with phosphate.
Lathanum itself is classified as a rare earth element. It's not actually that rare in deposits, it's just fairly difficult to mine and extract, hence why phosphate removers with lanthanum compounds are relatively expensive and largely unnecessary if proper chlorine levels are maintained.
Use when fighting Algae
When a user already has algae in their pool Phosphate Removers do very little. In fact when testing for phosphates with a green pool will result in a false result as quoted Algae in a Pool will result in a false reading.
This can be seen in the following link,
https://www.troublefreepool.com/threads/phosphates-debate.172875/#lg=post-1524456&slide=0Cite error: Closing </ref> missing for <ref> tag
Additionally a user may add a large amount of phosphate remover to the pool only to see phosphates return quickly once the SLAM is complete. This is because live algae has phosphates in the cell walls and these phosphates are released once the algae is killed. If a user removes all phosphates from their water before killing the algae there's a chance they'll need to do it again once their water is clear, thus wasting their money.
