What a viscious circle the pool stores have going.

Every opening/closing kit that I've seen where I live has - Oxidizer, 40% algaecide and "stain and scale" remover (which is the phosphonic acid). So you use this stuff at least twice a year (my pool store recommended algaecide and the stain and scale remover ONCE A MONTH), then they test for phosphates (which are increasing because of the phosphonic acid) and sell you phosphate remover.