The purple stuff is fine to use....so is metal free....Metal free is EDTA, a chelating agent. I have used both (along with some other ones)..they all work. My point was that phosphates are not a problem in a pool UNLESS you have recurrant algae that won't go away easily and phosphates are the 'limiting factor' in the algae problem (that means no nitrates and sanitizer levels are in correct amounts for the stabilizer level). In THAT case using a phosphate remover to reduce the phosphates in the water might help solve the algae problem...If there is no problem why try and fix one!