It's a carbon source, so bacteria consume it and the phosphate over time. Depending on the dosing rate, and the system, it might be weeks before the phosphate level changes much. How high is the phosphate level in the tank? How long has the phosphate been at that level? Live rock can adsorb phosphate and release it over time, which can be a problem for aquarists.
I'm very satisfied with this product. My tank was covered with nuisance algae and the starting of hair algae. Started dosing the beginning of the month and tank couldn'nt look any better. Just ordered reef energy A and B. Plan to use the whole redsea reef care program.