I've used only the Coral Pro version and it never tested for Po4 . It was tested with Salifert and the AP Po4 tablets . Since I cannot buy it locally anymore I have gone back to Oceanic . No regrets with the Pro Salt .
the red sea salt has very low alk for my taste i used it once and hated it.. it averages 7 maybe 8 at highest when freshly miced for me... i always had to baking soda it to make it right..