Most flake food is between 1 & 2 percent phosphate by weight.
Because of this I imported some Nutrafin Max flake from the US, it is a low phosphate flake with phosphate garuanteed between 0.6 & 0.9 percent.
I always read the label when buying fish food, if it does not say how much phosphate is in it, there is probably a reason they do not want to tell you.
But on the subject of washing frozen food, once a week or so I give my fish an all they can eat feed of mysis, I deliberately don't wash it because the corals obviously love all that fine mushy stuff, all the polyps come out to catch it, even the LPS's put their tentacles out for it, and even the zoos and palythoa can be seen catching it. Yes even zoos catch food they can be seen catching and eating it just like an anemone.