Originally from this thread, moved to The Deep End. JasonLion
Pool Perfect with Phos Free contains Lanthanum chloride and Lanthanum sulfate. Lanthanum is a silvery white metallic element. The metal reacts directly with elemental carbon, nitrogen, boron, selenium, silicon, phosphorus, sulfur, and with halogens.
As it reacts with the phosphate, the insoluble lanthanum phosphate precipitates out as a very fine white compound. The particles formed are too small to be removed with a conventional filter, and often require significant effort to remove. Further, if an excess of the compound is added, various side reactions may occur, including the formation of other precipitates that are similarly distributed throughout the pool, and difficult to remove. You can floc if necessary and then vacuum to waste.
Municipal water suppliers are using phosphates to help control corrosion in water pipelines. Their use of orthophosphates causes the most problems. Most available phosphate test kits measure the element in parts per billion (ppb). But make sure you have a test kit that measures orthophosphate, as it is the only form of phosphate algae can digest. The level of orthophosphate should be no higher than 125 ppb. Once levels get up above 200ppb, the likelihood of algae developing becomes much greater. More phosphate means the algae will be more resistant to typical treatments. Once your phosphates are under control, discontinue use of the Phos Free. Use a good Poly 60 algaecide on a regular basis to control algae.
You need to be careful that your cell does not become clogged. Check it regularly. It is important to avoid putting any chemicals through your skimmer while the cell is in-line.