In my attempt to control Iron staining I utilize Jack's Magic Purple Stuff which contains Phosphates. I have maintaining the pool, for some months now, after doing an in-pool AA treatment, at 20 ppm or above, and possibly up to up to 30 ppm, if I am busy that week and need a reserve (20 ppm is recommended by them to remove staining - recommended maintenance level is 10 ppm). I use their "Sequestrant Test Kit" to measure. The pH is maintained at 7.3 ppm and keeping the Calcium at the lower end for a plaster pool at 300 ppm also seems to help, as does back washing twice per month. A fair amount of clouding is visible when backwashing the first minute, my filter instructions states 2 to 3 minutes of backwashing, I do it for 2 minutes.
The pool for the first time ever in many years has remained stain free or at least no noticeable iron staining, normally after a few months at a maintenance level of 10 ppm stains start to reappear. With 20 ppm Plus of sequestrant even my 2 long steps about 17 1/2 feet long have remained stain free.
Out of curiosity I checked the Phosphate level today, with a Phosphate test powder, using 1/24 the amount of water in the test tube with 23/24 tap water (our tap water registers zero for Phosphates). The result was half way in color between 1,000 and 500 ppm, so 24 times say 750 would be 18,000 ppm of Phosphates. So it seems that, as often mentioned on this site, that Phosphates if the Chlorine is held at the proper levels are indeed no problem or issue. I appreciate these Phosphate numbers are very rough but I think close enough.
One issue with the "Sequestrant Test Kit" is that the new tablets which are foil wrapped do not instantly mix, so it is better to mix then let it settle for 5 or 10 minutes, and then mix again, then shake, covering the pin hole opening on the cover, and then test. If this is not done as all of the tablet has not dissolved one will get a false reading indicating a higher amount of sequestrant in the pool than is really there, example: if not mixed properly reading indicates 18 ppm of sequestrant in the pool, if mixed properly indicates 10 ppm of sequestrant in the pool. Also rocking the test tube back and forth, say 45 degrees in each direction (not upside down) gently helps to mix the test sequestrant better and gives more accurate results.
I really believe that the "Sequestrant Test Kit" is an invaluable tool if one is trying to control iron staining, otherwise one is walking in the dark as to how much sequestrant is in the pool. If the winds are moderate I test once a week, if there are high winds say over 30 mph I will test twice per week. The reason for this is my iron is wind born volcanic dust which contains iron, the fill water does not.
A small tip; even after doing aa AA treatment the return eye jets (as the eye is turnable and in my case has grooves on the edge) still may show some brown/orange staining. Unscrew them and put them in a small bowl or glass which has one or two unbuffered Vitamin C tablets,which are fully dissolved and mixed, this may take an hour or two. Add a drop or two of white vinegar, then soak them for about 10 minutes, or longer if necessary, finally flushing them with tap water. I do one eye at a time so that I put them back in the same place. Also a good indication that you may not have enough sequestrant in the pool is to look at your white thermometer if it is staining then you may need to add sequestrant. You can clean your thermometer the same way as the eyes are cleaned.
Hope this helps.