can't get any FC after putting in a lot of Cl... phosphate?

Hi all. Had a new filter/pump installed this year and the company opened the pool (25,000 gal in ground plaster pool). They only added metal (iron?) remover. It was cloudy after they opened and brushed the poo but has since cleared up. l put in 11 bags of shock... no FC read. I put in 8 gal of 11% bleach... 0.5 FC read. Took a water sample into the pool supply store. They tested it right away for phosphate... the water turned a light blue and they said it was high phosphate that was binding with the Cl. They sold me a bottle of phosphate remover ($50). Before I use it, does that make sense? Not sure where phosphate is coming from and I don't understand the chemistry of Cl binding with the phosphate. Thanks so much!