My experience was different. My pool is 13,500 gallon IG, and my phosphates tested at 2500+. 4 days after the 1st 2l bottle of phos-free, I was down to 300-500. I added a bottle of salt-water magic, and it's now down to 100-200. Did you leave the pho-free in the filter long enough? My bottle said not to back flush for at least 2 days. My filter ia a cartridge, so the phos free will be in there until the next time I service it, which may be a while. I did not see a pressure rise from the phos-free

On thing worth mentioning was that I held the pH at 7.2-7.4 during the treatment and I shocked to 20 ppm Cl to kill the algae at the same time.